Currently proto symbolization uses runtime.FuncForPC and assumes each
PC maps to a single frame. This isn't true in the presence of inlining
(even with leaf-only inlining this can get incorrect results).

Change PC symbolization to use runtime.CallersFrames to expand each PC
to all of the frames at that PC.

Change-Id: I8d20dff7495a5de495ae07f569122c225d433ced
Reviewed-on: https://go-review.googlesource.com/41256
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>