<third_party/angle> Fix crash in TSymbolTableLevel::~TSymbolTableLevel with gcc 4.9

WebGL crashes when chromium is compiled with gcc 4.9,
because TSymbolTableLevel::~TSymbolTableLevel does
double delete of e.g. TFunctions because they are
inserted in the table with both their name and
mangled name.

The fix for this should reach stable with m37.
https://code.google.com/p/angleproject/issues/detail?id=651

Patch by Mike Hommey <mh+mozilla@glandium.org>
https://bugzilla.mozilla.org/show_bug.cgi?id=1025576

Change-Id: I0571478759666bcb55633dd4e630c5b42216d949
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>