GTK: First cut at tab dragging automation.

Also make tab dragging slightly more robust.

I tried really hard to avoid hackiness, but after many hours of wrestling with gtk and X, this is the best I could do. The main point of contention is that GTK (and our tab dragging code in particular) seems to be able to get X into a state where gdk_display_warp_pointer() doesn't send back any events (although it does move the X pointer). I tried to fix our code directly, but decided it was GTK that was broken. So I faked some mouse motion events to prod the tab dragging into working. This approach does not appear to be flaky, and is actually closer to the event stream that occurs when a user drags a tab than the obvious approach would be. (The tests themselves are somewhat flaky, but only due to WaitForURLDisplayedForTab() flakiness, which is a separate issue I'll look at later. The tests aren't on any buildbot for now so I'd like to leave them enabled.)

BUG=22182
TEST=--gtest_filter=AutomatedUITest.Drag*

Review URL: http://codereview.chromium.org/218017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@27166 0039d316-1c4b-4281-b951-d872f2087c98