Safely cancel prerenders on threads other than the UI thread.

Previously, prerendering was cancelled on the IO thread by not
doing something, and then passing a task to the UI thread
to abort the prerender.  This resulted in a race which could
result in swapping in the prerender before the task was
executed.  This fixes that.

BUG=83062
TEST=PrerenderStatusManagerTests, PrerenderBrowserTests
Review URL: http://codereview.chromium.org/7038012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@86082 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/chrome/chrome_browser.gypi b/chrome/chrome_browser.gypi
index 5465fc8..d1c0605 100644
--- a/chrome/chrome_browser.gypi
+++ b/chrome/chrome_browser.gypi
@@ -1653,6 +1653,8 @@
         'browser/prerender/prerender_render_view_host_observer.h',
         'browser/prerender/prerender_render_widget_host_view.cc',
         'browser/prerender/prerender_render_widget_host_view.h',
+        'browser/prerender/prerender_tracker.cc',
+        'browser/prerender/prerender_tracker.h',
         'browser/printing/background_printing_manager.cc',
         'browser/printing/background_printing_manager.h',
         'browser/printing/cloud_print/cloud_print_proxy_service.cc',