Expanding on point #1. Presently, when symbolize=0, we use a dummy symbolizer which
returns 0 symbolized frames. POSIXSymbolizer will fall back to module+offset if the
on-line symbolizer fails, but the dummy symbolizer won't. We don't normally notice
that, because StackTrace::PrintStack reimplements this fallback mechanism. Hence, we
see module+offset in reports even when symbolize=0, but suppression matching (which
doesn't use PrintStack()) won't work. This is bad - in some environments we don't have
the on-line symbolizer, but we still want to skip, e.g., the hundreds of false positives
coming from libfontconfig in Chrome.

Possible extra improvement: with symbolize=1, support a way to get just the module
name first, so we won't have to symbolize if the module name is suppressed (this should
improve performance somewhat).

Also, vis-a-vis #2, I'm guessing the use of symbolize callback needs to be moved from
PrintStack() to the symbolization code as well?

After the recent changes:
* We create a POSIXSymbolizer (and thus get <module>+<offset>) even if symbolize=0.
So, suppression by module name should now always work. In fact, we have one extra path
in PrintStackTrace which is not present in Symbolizer - fall back to /proc/self/maps
if symbolizer failed to return anything. Do you think we should move this functionality
to Symbolizer as well?
* symbolize callback functionality is removed.
* It is possible to add a method Symbolizer::GetPCModuleAndOffset(), but I'd like to
avoid it unless we see a measurable performance improvement.

> Do you think we should move this functionality to Symbolizer as well?

I don't see why not.

This should be fixed at r198025 by a series of recent commits. Currently StackTrace::PrintStack
is pretty straightforward, there is no symbolize callback and no /proc/self/maps fallback.
Symbolizer::GetModuleNameAndOffsetForPC() is implemented and is used in LSan suppression
matcher.

Adding Project:AddressSanitizer as part of GitHub migration.