The extension icon on the far right is invisible when about to dragging.

The browser action button on the far right is invisible when user will
be dragging the browser actions container.

Because the browser action buttons, which don't locate at the extension
bar, will be set to be hidden when the dragging finished. So the hidden
buttons must be set to be visible before the dragging starts in the next
time in order to let user know which extension is coming.

BUG=424080
TEST=manually tested.

Review URL: https://codereview.chromium.org/662593002

Cr-Commit-Position: refs/heads/master@{#300049}

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

@@ -637,7 +637,9 @@ class ExtensionServiceObserverBridge
 - (void)containerDragStart:(NSNotification*)notification {
   [self setChevronHidden:YES inFrame:[containerView_ frame] animate:YES];
   while([hiddenButtons_ count] > 0) {
-    [containerView_ addSubview:[hiddenButtons_ objectAtIndex:0]];
+    BrowserActionButton* button = [hiddenButtons_ objectAtIndex:0];
+    [button setAlphaValue:1.0];
+    [containerView_ addSubview:button];
     [hiddenButtons_ removeObjectAtIndex:0];
   }
 }