Update content/renderer code to use WebElementCollection instead of WebNodeCollection

Update content/renderer code to use WebElementCollection instead of WebNodeCollection.
Update safe_browsing code to use WebElementCollection instead of WebNodeCollection.
After Blink r166411 and r166500, WebNodeCollection is renamed to
WebElementCollection and its API returns WebElements instead of WebNodes.

The new code is clearer and avoids unnecessary isElementNode() checks.

Review URL: https://codereview.chromium.org/149913011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249736 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249736