Add instrumentation to track down a crash.
Add instrumentation to track down a crash.

Promotes some DCHECKs to CHECKs, to figure out who is calling URLFetcher::Start() without having first set a valid request context.

Also adds a |was_cancelled_| variable to rule out that Start() isn't called after the request was already cancelled.

This change can be reverted once we get the data.

BUG=27074

Review URL: http://codereview.chromium.org/377041

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@31526 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser