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Due to a recent cl, URLRequest must get a non-NULL context in the constructor.
This means that if-statements and DCHECKs that verify the existence of context
are no longer needed.

BUG=None


Review URL: https://chromiumcodereview.appspot.com/10830124

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@151326 0039d316-1c4b-4281-b951-d872f2087c98