Mac: Fix window border appearance when tab dragging.
As written in -[TabStripDragController setWindowBackgroundVisibility:],
dragWindow_ should be invisible and the overlayWindow should have a shadow
when there is a |targetController_|.
Fix a case when this state is out of sync right after creation of the dragged
window.
BUG=517447
TEST=Open 2 tabs. Drag a tab to horizontally to the right until it detached
while still targetting its current window. The dragged window should now have
a border and shadow.
Review URL: https://codereview.chromium.org/
1307183005
Cr-Commit-Position: refs/heads/master@{#346202}