<webview>: Disable navigation on drag-and-drop.

This behavior is undesired because developers may want <webview>s to stay
at the URL they assigned it.

The Blink side change can be found here: https://codereview.chromium.org/147493016/

BUG=342394
TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/159203002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252396 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252396