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When the DomDistillerViewerSource got a request for chrome-distiller://foobar/
it would crash if there was an empty path argument. This CL adds a safety check
for the length of the path, to ensure to not take a substring of an empty
string. 

In addition, this adds regression tests for the crash it fixes. It also fixes
the rest of the disabled browser tests for the DomDistillerViewerSource. 

BUG=356866

Review URL: https://codereview.chromium.org/288353002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271178 0039d316-1c4b-4281-b951-d872f2087c98