Don't DCHECK being on the UI thread in failed callback handler.

If a thumbnail request fails because the browser is shutting down. In
that case the failure may be executed after the browser UI thread
is being closed, and the CurrentlyOn(content::BrowserThread::UI)
check will fail.

So move this check to only be executed when the ProcessCapturedBitmap
result is successful, and the function is going to do any work.

R=sky
BUG=270918

Review URL: https://codereview.chromium.org/141273011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245767 0039d316-1c4b-4281-b951-d872f2087c98
[Telemetry] Dismiss crash dialog before starting Chrome.

After a Chrome crash, a dialog will cover a portion of the screen. This prevents
speed index from working properly.

BUG=323813

Review URL: https://codereview.chromium.org/141723003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245768 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245768