Fix for the Layout test failures occuring as a result of the change to download the plugin src URL in a background task. The tests fail because the plugin instance is torn down before the task executes. 
Fix for the Layout test failures occuring as a result of the change to download the plugin
src URL in a background task. The tests fail because the plugin instance is torn down before
the task executes. 

The fix is to revoke tasks executing on the current WebPluginImpl instance when it is torn down.

TBR=jam
Bug=12993

Review URL: http://codereview.chromium.org/119410

git-svn-id: http://src.chromium.org/svn/trunk/src@18035 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 9ad63fd616330f30521b95c78b61e00a9d972359
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser