This is a modified https://codereview.chromium.org/116143003/. The original patch was reverted (https://codereview.chromium.org/134873009) because enumerating shortcuts on the FILE thread was a huge task and it wasn't cancelled on browser shutdown. Now it's cancelable. The task is cancelled if the corresponding ResettableSettingsSnapshot destroyed. It happens if user closes chrome://settings tab.

BUG=324931
TBR=jyasskin@chromium.org

Review URL: https://codereview.chromium.org/137533010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246705 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246705