Disable contents border for shared tab on Mac.

Drive-by fix crash when the browser is closed while sharing a tab.

Bug: 991896
Change-Id: I00fe5791cd6856f8258a279a7fa5f6e7d276237f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1746169
Commit-Queue: Marina Ciocea <marinaciocea@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685533}

chrome/browser/ui/views/tab_sharing/tab_sharing_ui_views.cc

@@ -30,6 +30,7 @@
 
 namespace {
 
+#if !defined(OS_MACOSX)
 const int kContentsBorderThickness = 10;
 const float kContentsBorderOpacity = 0.24;
 const SkColor kContentsBorderColor = gfx::kGoogleBlue500;
@@ -68,8 +69,11 @@ void InitContentsBorderWidget(content::WebContents* contents) {
 
   browser_view->set_contents_border_widget(widget);
 }
+#endif
 
 void SetContentsBorderVisible(content::WebContents* contents, bool visible) {
+  if (!contents)
+    return;
   Browser* browser = chrome::FindBrowserWithWebContents(contents);
   if (!browser)
     return;
@@ -108,7 +112,10 @@ TabSharingUIViews::TabSharingUIViews(const content::DesktopMediaID& media_id,
           media_id.web_contents_id.main_render_frame_id));
   shared_tab_name_ = GetTabName(shared_tab_);
   profile_ = ProfileManager::GetLastUsedProfileAllowedByPolicy();
+#if !defined(OS_MACOSX)
+  // TODO(https://crbug.com/991896) fix contents border on Mac.
   InitContentsBorderWidget(shared_tab_);
+#endif
 }
 
 TabSharingUIViews::~TabSharingUIViews() {