[android_webview] Suppress warning for Android build.

This suppresses a warning in the Android build configuration.

BUG=None
TEST=compile
NOTRY=true
R=torne@chromium.org

Review URL: https://codereview.chromium.org/149813003

git-svn-id: http://src.chromium.org/svn/trunk/src/build@250726 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -3852,6 +3852,7 @@
                   '-Wno-extra', # Enabled by -Wextra, but no specific flag
                   '-Wno-ignored-qualifiers',
                   '-Wno-type-limits',
+                  '-Wno-unused-but-set-variable',
                 ],
                 'cflags_cc': [
                   # Other things unrelated to -Wextra: