Disable -Wmismatched-tags for mesa.

We build with 3rd-party code with -Wall on the clang/win bot. This has already
been fixed upstream, so there's no need to keep this around

https://github.com/grate-driver/mesa/commit/68a4bb553bcd51d336d95b0c845c0dc63adc3bd3

BUG=none
R=hans@chromium.org
TBR=cpu

Review URL: https://codereview.chromium.org/470313002

Cr-Commit-Position: refs/heads/master@{#289680}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289680 0039d316-1c4b-4281-b951-d872f2087c98

third_party/mesa/mesa.gyp

@@ -132,6 +132,7 @@
       'variables': {
         'clang_warning_flags': [
           '-Wno-tautological-constant-out-of-range-compare',
+          '-Wno-mismatched-tags',  # Fixed upstream.
         ],
         'clang_warning_flags_unset': [
           # Don't warn about string->bool used in asserts.
@@ -270,6 +271,7 @@
         'clang_warning_flags': [
           '-Wno-tautological-constant-out-of-range-compare',
           '-Wno-absolute-value',  # Fires on st_atom_array.c, might be a bug
+          '-Wno-mismatched-tags',  # Fixed upstream.
         ],
         'clang_warning_flags_unset': [
           # Don't warn about string->bool used in asserts.