Clear the download item's target on dealloc.

This fixes a crash if the download item controller goes away while the
button is tracking the mouse and then sends its action on mouse up.

Bug: 815161
Change-Id: I4ab377ca5fe190e3d95aa81b989c13c96b46f5b4
Reviewed-on: https://chromium-review.googlesource.com/957843
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542357}

chrome/browser/ui/cocoa/download/download_item_controller.mm

@@ -122,6 +122,7 @@ class DownloadShelfContextMenuMac : public DownloadShelfContextMenu {
 - (void)dealloc {
   [[NSNotificationCenter defaultCenter] removeObserver:self];
   [progressView_ setController:nil];
+  [progressView_ setTarget:nil];
   [[self view] removeFromSuperview];
   [super dealloc];
 }