Remove calls of PrerenderTracker::TryCancelOnIOThread in SafeBrowsingResourceThrottle and instead PrerenderContents::Destroy directly on the UI thread.

This is part of the work on removing calls to ResourceRequestInfo::GetAssociatedRenderView. The last callers to that method are using it to cancel prerenders if they haven't been used yet, so the checking/cancelling needs to all move to the UI thread.

BUG=304341
R=davidben@chromium.org

Review URL: https://codereview.chromium.org/103633008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243063 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243063
Add svn:ignore for third_party/brotli.

BUG=330462
TEST=None



git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243064 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243064