Turn on prefetching in webkit builds

After http://codereview.chromium.org/2847093/show lands, I'll land
this CL which modifies our build controls for webkit to turn on
prefetching.

This change might require builders to be clobbered, and by making it
so small, I'm sure a sherriff will see what's up.  However, I don't
believe it will actually require a clobber, since there's no unit
tests for prefetching (only functional tests and dumprendertree tests
in webkit).

TBR=tonyg@chromium.org
BUG=none
TEST=http://gemal.dk/browserspy/prefetching.php

Review URL: http://codereview.chromium.org/3080015

git-svn-id: http://src.chromium.org/svn/trunk/src/build@54509 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

features_override.gypi

@@ -28,7 +28,7 @@
       'ENABLE_INPUT_SPEECH=1',
       'ENABLE_JAVASCRIPT_DEBUGGER=1',
       'ENABLE_JSC_MULTIPLE_THREADS=0',
-      'ENABLE_LINK_PREFETCH=0',
+      'ENABLE_LINK_PREFETCH=1',
       'ENABLE_METER_TAG=1',
       'ENABLE_NOTIFICATIONS=1',
       'ENABLE_OFFLINE_WEB_APPLICATIONS=1',