Fix <webview> crash when drag-and-drop is initiated on GTK

BUG=305274
Test=Load Browser sample app on Linux GTK build, drag some text outside of the webview, perhaps outside of the window. The webview will crash.

Review URL: https://codereview.chromium.org/26168006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227676 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227676