Change PhishingDOMFeatureExtractor to cache the WebDocument rather than a WebFrame pointer.

      WebFrames are not guaranteed to remain valid across calls to ExtractFeaturesWithTmeout.

BUG=97148
TEST=PhishingDOMFeatureExtractorTest.SubframeRemoval


Review URL: http://codereview.chromium.org/7976039

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@102541 0039d316-1c4b-4281-b951-d872f2087c98