Add -Wno-sign-compare flag to gcc builds.

Fix gcc 4.9.2 signed-compare warnings/errors.

BUG=

Review URL: https://codereview.chromium.org/1476473002

Cr-Commit-Position: refs/heads/master@{#32367}

build/standalone.gypi

@@ -406,6 +406,9 @@
       ],
     },
     'conditions':[
+      ['clang==0', {
+        'cflags+': ['-Wno-sign-compare',],
+      }],
       ['(clang==1 or host_clang==1) and OS!="win"', {
         # This is here so that all files get recompiled after a clang roll and
         # when turning clang on or off.