[Mac] Fix bug where sometimes a tab's close button stays highlighted.

The problem was caused by the fact that tracking was set to
disabled (which happens when the mouse leaves the tab strip),
which made future -checkImageState: calls early return, and
thus the button stayed highlighted.

Easiest repro steps for the bug I found (which still requires
a few tries to get right) are as follows:
  - Open exactly three tabs.
  - Close the middle tab by clicking on it's close button.
  - Right after the click move your mouse diagonally down-right,
    out of the tab strip, passing by the close button of the last
    tab which is animating into the middle tab's position.
  - On a succesful repro, your mouse is out of the tab strip, but
    the second tab's close button is now highlighted.

With this fix, the above steps do not repro the bug. Also verified
with print statements that output the flow of events.

BUG=433491
TEST=See descriptions above.

Review URL: https://codereview.chromium.org/720413004

Cr-Commit-Position: refs/heads/master@{#304319}

ui/base/cocoa/hover_button.mm

@@ -78,6 +78,7 @@ - (void)setTrackingEnabled:(BOOL)enabled {
                afterDelay:0];
   } else {
     if (trackingArea_.get()) {
+      self.hoverState = kHoverStateNone;
       [self removeTrackingArea:trackingArea_.get()];
       trackingArea_.reset(nil);
     }