Refcount notification delegates before use.

This prevents certain classes of bugs where delegates are deleted
during the execution of certain callback methods.

BUG=336650

Review URL: https://codereview.chromium.org/151683002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250283 0039d316-1c4b-4281-b951-d872f2087c98
Update android WebView to use WebElementCollection instead of WebNodeCollection

Update android WebView 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 an unnecessary isElementNode() check.

Review URL: https://codereview.chromium.org/149313005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250284 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250284