[Cocoa] Crash fix, caused by recent tab indicator change.

I misunderstood how ObjectiveC handled pointers declared with __weak.
This change resolves the issue by nil'ing things out in
[TabController dealloc], the standard solution.

Introduced by: https://codereview.chromium.org/688523002/

BUG=429605

Review URL: https://codereview.chromium.org/699013002

Cr-Commit-Position: refs/heads/master@{#302536}

chrome/browser/ui/cocoa/tabs/tab_controller.mm

@@ -128,6 +128,8 @@ class MenuDelegate : public ui::SimpleMenuModel::Delegate {
 }
 
 - (void)dealloc {
+  [mediaIndicatorButton_ setAnimationDoneTarget:nil withAction:nil];
+  [mediaIndicatorButton_ setClickTarget:nil withAction:nil];
   [[NSNotificationCenter defaultCenter] removeObserver:self];
   [[self tabView] setController:nil];
   [super dealloc];