Remove the intent to turn on Wextra for gcc (except MacOS).

We use clang with Wextra which seems to be more reliable and report less false positives. We no longer have an intent to turn on Wextra for gcc on posix-like systems any more (except MacOS).

Review URL: https://codereview.chromium.org/1029953003

Cr-Commit-Position: refs/heads/master@{#322101}

build/common.gypi

@@ -3603,8 +3603,6 @@
           '-pthread',
           '-fno-strict-aliasing',  # See http://crbug.com/32204
           '-Wall',
-          # TODO(evan): turn this back on once all the builds work.
-          # '-Wextra',
           # Don't warn about unused function params.  We use those everywhere.
           '-Wno-unused-parameter',
           # Don't warn about the "struct foo f = {0};" initialization pattern.