Allow symbolz to try symbolize unsymbolizable mappings.

We added skipping unsymbolizable mappings in symbolz long time ago in
PR google#368 to address google#339 where we saw error like "unexpected negative
adjusted address".

But that error was fixed in a more proper way in subsequent google#397 to fix
another reported issue google#280 (and internal b/32020573). So skipping
unsymbolized mappings shouldn't be needed anymore.

I tested this by verifying that the test case from google#339 still works fine
with the proposed change. And that it does fail if I roll back google#397
locally.

This change is useful as we experiment with using symbolz to symbolize
JIT locations from //anon (which is unsymbolizable per current terminology).

internal/symbolz/symbolz.go

@@ -37,19 +37,15 @@ var (
 // Symbolize symbolizes profile p by parsing data returned by a symbolz
 // handler. syms receives the symbolz query (hex addresses separated by '+')
 // and returns the symbolz output in a string. If force is false, it will only
-// symbolize locations from mappings not already marked as HasFunctions. Never
-// attempts symbolization of addresses from unsymbolizable system
-// mappings as those may look negative - e.g. "[vsyscall]".
+// symbolize locations from mappings not already marked as HasFunctions. Does
+// not skip unsymbolizable files since the symbolz handler can be flexible
+// enough to handle some of those cases such as JIT locations in //anon.
 func Symbolize(p *profile.Profile, force bool, sources plugin.MappingSources, syms func(string, string) ([]byte, error), ui plugin.UI) error {
 	for _, m := range p.Mapping {
 		if !force && m.HasFunctions {
 			// Only check for HasFunctions as symbolz only populates function names.
 			continue
 		}
-		// Skip well-known system mappings.
-		if m.Unsymbolizable() {
-			continue
-		}
 		mappingSources := sources[m.File]
 		if m.BuildID != "" {
 			mappingSources = append(mappingSources, sources[m.BuildID]...)