feat(bazel): expose toolchain aliases for exposed browsers

Exposes toolchain alias targets for all exposed browsers by the
dev-infra Bazel package. i.e. Chromium and Firefox. These can then
be used within integration tests, or other e.g. webdriver tests using
Bazel make variable expansion.

bazel/browsers/chromium/BUILD.bazel

@@ -1,4 +1,5 @@
 load("@io_bazel_rules_webtesting//web:web.bzl", "browser")
+load("//bazel/browsers:browser_toolchain_alias.bzl", "browser_toolchain_alias")
 
 package(default_visibility = ["//visibility:public"])
 
@@ -27,6 +28,16 @@ browser(
     }),
 )
 
+browser_toolchain_alias(
+    name = "toolchain_alias",
+    metadata = select({
+        "//bazel/constraints:linux_x64": "@org_chromium_chromium_linux_x64//:metadata",
+        "//bazel/constraints:macos_x64": "@org_chromium_chromium_macos_x64//:metadata",
+        "//bazel/constraints:macos_arm64": "@org_chromium_chromium_macos_arm64//:metadata",
+        "//bazel/constraints:windows_x64": "@org_chromium_chromium_windows//:metadata",
+    }),
+)
+
 # Make source files available for distribution via pkg_npm
 filegroup(
     name = "files",

bazel/browsers/firefox/BUILD.bazel

@@ -1,4 +1,5 @@
 load("@io_bazel_rules_webtesting//web:web.bzl", "browser")
+load("//bazel/browsers:browser_toolchain_alias.bzl", "browser_toolchain_alias")
 
 package(default_visibility = ["//visibility:public"])
 
@@ -34,6 +35,16 @@ browser(
     }),
 )
 
+browser_toolchain_alias(
+    name = "toolchain_alias",
+    metadata = select({
+        "//bazel/constraints:linux_x64": "@org_mozilla_firefox_linux_x64//:metadata",
+        # Firefox has a launcher that conditionally starts x64/arm64
+        "//bazel/constraints:macos_x64": "@org_mozilla_firefox_macos//:metadata",
+        "//bazel/constraints:macos_arm64": "@org_mozilla_firefox_macos//:metadata",
+    }),
+)
+
 # Make source files available for distribution via pkg_npm
 filegroup(
     name = "files",