Remove CanChallengePlatform() and CheckPlatformState().

This API did not meet its intended purpose.  We've decided to instead
just use a real challenge and wait for the response.

BUG=307707,309158
TEST=compile, unittests pass.
TBR=pastarmovj

Review URL: https://codereview.chromium.org/29943003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229648 0039d316-1c4b-4281-b951-d872f2087c98
Add additional text to chrome://downloads warning confirmation

This adds more descriptive text to the chrome://downloads confirmation
for known malware.

BUG=305387

Review URL: https://codereview.chromium.org/30053003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229649 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229649