Reland #7 of "Force Python 3 to be used in build."

This is a reland of d9caee7.

A number of fixes have landed in between then and now, and I
think we're ready for another round.

Original change's description:
> Reland #6 of "Force Python 3 to be used in build."
>
> This reverts commit b2446a0
>
> Reason for revert: Trying again w/ build fixes.
>
> The two issues we saw yesterday with reland attempt #5
> were fixed in
>   https://crrev.com/c/2768998 (remoting linux installer)
>   https://crrev.com/c/2768998 (GLES test scripts)
>
> But the change wasn't in the tree for very long, so there may
> well be more. Let's try again to see.
>
> Bug: 1112471
> Cq-Include-Trybots: luci.chromium.try:linux-angle-rel
> Cq-Include-Trybots: luci.chrome.try:linux-chrome
> Change-Id: Ie5483d1f6838deeaf0f9b7cddc0bb57a82960c29
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2769361
> Bot-Commit: Rubber Stamper <[email protected]>
> Commit-Queue: Dirk Pranke <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#864372}

Bug: 1112471
Change-Id: Iccef0aa5d92c28fee1ce67e446b9b15a7cc0745a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2792623
Commit-Queue: Dirk Pranke <[email protected]>
Bot-Commit: Rubber Stamper <[email protected]>
Reviewed-by: Bruce Dawson <[email protected]>
Cr-Commit-Position: refs/heads/master@{#868187}

.gn

@@ -8,6 +8,10 @@ import("//third_party/angle/dotfile_settings.gni")
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
+# The python interpreter to use by default. On Windows, this will look
+# for python3.exe and python3.bat.
+script_executable = "python3"
+
 # These arguments override the default values for items in a declare_args
 # block. "gn args" in turn can override these.
 #

tools/mb/mb.py

@@ -1090,33 +1090,6 @@ def RunGNGen(self, vals, compute_inputs_for_analyze=False, check=True):
       self.WriteFile(gn_runtime_deps_path, '\n'.join(labels) + '\n')
       cmd.append('--runtime-deps-list-file=%s' % gn_runtime_deps_path)
 
-    # Detect if we are running in a vpython interpreter, and if so force GN to
-    # use the real python interpreter. crbug.com/1049421
-    # This ensures that ninja will only use the real python interpreter and not
-    # vpython, so that any python scripts in the build will only use python
-    # modules vendored into //third_party.
-    # This can be deleted when python 3 becomes the only supported interpreter,
-    # because in python 3 vpython will no longer have its current 'viral'
-    # qualities and will require explicit usage to opt in to.
-    prefix = getattr(sys, "real_prefix", sys.prefix)
-    python_exe = 'python.exe' if self.platform.startswith('win') else 'python'
-    # The value of prefix varies. Sometimes it extends to include the bin/
-    # directory of the python install such that prefix/python is the
-    # interpreter, and other times prefix/bin/python is the interpreter.
-    # Therefore we need to check both. Also, it is safer to check prefix/bin
-    # first because there have been previous installs where prefix/bin/python
-    # was the real binary and prefix/python was actually vpython-native.
-    possible_python_locations = [
-        os.path.join(prefix, 'bin', python_exe),
-        os.path.join(prefix, python_exe),
-    ]
-    for p in possible_python_locations:
-      if os.path.isfile(p):
-        cmd.append('--script-executable=%s' % p)
-        break
-    else:
-      self.Print('python interpreter not under %s' % prefix)
-
     ret, output, _ = self.Run(cmd)
     if ret != 0:
       if self.args.json_output: