--force-compositing-mode on TOUCH_UI

We've got two texture transport paths that cover a wide range of hardware/driver configurations, so it's time to turn this on by default for TOUCH_UI.

BUG=none
TEST=run with --show-fps-counter on www.google.com to verify that simple pages use WebKit compositor

Review URL: http://codereview.chromium.org/7748040

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@98454 0039d316-1c4b-4281-b951-d872f2087c98

content/browser/tab_contents/tab_contents.cc

@@ -1799,6 +1799,10 @@ WebPreferences TabContents::GetWebkitPrefs() {
       web_prefs.accelerated_compositing_enabled = false;
 #endif
 
+#if defined(TOUCH_UI)
+  web_prefs.force_compositing_mode = true;
+#endif
+
   return web_prefs;
 }