[Extensions UI Mac] Use frameAfterAnimation for extension action butt…

…on opacity

Button opacity for extension actions is set when the container frame changes,
but if the button is also animating, we need to use the final frame to determine
the proper opacity.

BUG=530820

Review URL: https://codereview.chromium.org/1346443002

Cr-Commit-Position: refs/heads/master@{#348696}

chrome/browser/ui/cocoa/extensions/browser_actions_controller.mm

@@ -668,7 +668,7 @@ - (BOOL)updateContainerVisibility {
 
 - (void)updateButtonOpacity {
   for (BrowserActionButton* button in buttons_.get()) {
-    NSRect buttonFrame = [button frame];
+    NSRect buttonFrame = [button frameAfterAnimation];
     if (NSContainsRect([containerView_ bounds], buttonFrame)) {
       if ([button alphaValue] != 1.0)
         [button setAlphaValue:1.0];