cocoa: reset observer in HungRendererController dealloc

HungRendererController autoreleases itself, which doesn't work well when it
doesn't actually hold a reference to the containing window (which it doesn't
in MacViews builds). This fixes the HungRendererController interactive UI test.

Bug: 823517
Change-Id: I5fc14faa8eeaba7b20f5e922a43d4817ca2c810a
Reviewed-on: https://chromium-review.googlesource.com/1015261Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551412}

chrome/browser/ui/cocoa/hung_renderer_controller.mm

@@ -261,6 +261,13 @@ class HungRendererObserverBridge : public content::WebContentsObserver,
   hungContents_ = nullptr;
   hungWidget_ = nullptr;
 
+  // Reset the observer now. It is not necessarily the case that this class
+  // actually holds a reference to the containing BrowserWindow, and if it does
+  // not, the BrowserWindow's destructor can run *before* this object is cleaned
+  // up by the autorelease pool. This can't happen in practice, but it can
+  // happen in tests.
+  hungContentsObserver_.reset();
+
   [self autorelease];
 }