Guard access to ContentClient in occlusion checker of Mac

Ensure that `performOcclusionStateUpdates` and `setWebContentsVisibility` check if content::GetContentClient() is nullptr before proceeding. Add necessary safeguards to prevent the crash when content::GetContentClient() is nullptr.

Bug: 385901565
Change-Id: I336982157f115517343db38223e4aabdcb6afa89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6110550
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Leon Han <shulianghan@microsoft.com>
Commit-Queue: Lichen Liu <lichenliu@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1403344}

content/app_shim_remote_cocoa/web_contents_occlusion_checker_mac.mm

@@ -383,8 +383,12 @@ bool IsBrowserProcess() {
 - (void)performOcclusionStateUpdates {
   _occlusionStateUpdatesAreScheduled = NO;
 
-  if (content::GetContentClient()->browser() &&
-      content::GetContentClient()->browser()->IsShuttingDown()) {
+  auto* content_client = content::GetContentClient();
+  if (!content_client) {
+    return;
+  }
+  auto* browser = content_client->browser();
+  if (browser && browser->IsShuttingDown()) {
     return;
   }
 

content/app_shim_remote_cocoa/web_contents_view_cocoa.mm

@@ -403,10 +403,18 @@ STATIC_ASSERT_ENUM(NSDragOperationMove, ui::DragDropTypes::DRAG_MOVE);
 }
 
 - (void)setWebContentsVisibility:(remote_cocoa::mojom::Visibility)visibility {
-  if (_host && !(content::GetContentClient()->browser() &&
-                 content::GetContentClient()->browser()->IsShuttingDown())) {
-    _host->OnWindowVisibilityChanged(visibility);
+  if (!_host) {
+    return;
   }
+  auto* content_client = content::GetContentClient();
+  if (!content_client) {
+    return;
+  }
+  auto* browser = content_client->browser();
+  if (browser && browser->IsShuttingDown()) {
+    return;
+  }
+  _host->OnWindowVisibilityChanged(visibility);
 }
 
 - (void)performDelayedSetWebContentsOccluded {