HttpAuthHandlerDigest unit test cleanup.
HttpAuthHandlerDigest unit test cleanup.

DCHECK replaced by EXPECT for HttpAuthHandlerDigest unittests.

Also, changed RespondToChallenge to return a success boolean and LOG the failure reasons, rather than returning an error string. 

BUG=None
TEST=net_unittests --gtest_filter="*HttpAuthHandlerDigest*"
Review URL: http://codereview.chromium.org/5205005

git-svn-id: http://src.chromium.org/svn/trunk/src@68006 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 16d763617bb21f957333b45aa005ffee26c7f88f
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser