Fix 2 access violation in resource_coordinator.

Bug: 818454
Change-Id: I77f6ea60f38ad8956d7f7e042c57202f3f41a07c
Reviewed-on: https://chromium-review.googlesource.com/951749
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#541230}(cherry picked from commit 182f838)
Reviewed-on: https://chromium-review.googlesource.com/953366
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/branch-heads/3359@{#73}
Cr-Branched-From: 66afc5e-refs/heads/master@{#540276}

Author: sebmarchand

chrome/browser/resource_coordinator/tab_lifecycle_unit_source.cc

@@ -161,7 +161,11 @@ void TabLifecycleUnitSource::TabChangedAt(content::WebContents* contents,
   if (change_type != TabChangeType::kAll)
     return;
   auto it = tabs_.find(contents);
-  DCHECK(it != tabs_.end());
+  // The WebContents destructor might cause this function to be called, at this
+  // point TabClosingAt has already been called and so this WebContents has
+  // been removed from |tabs_|.
+  if (it == tabs_.end())
+    return;
   TabLifecycleUnit* lifecycle_unit = it->second.get();
   lifecycle_unit->SetRecentlyAudible(contents->WasRecentlyAudible());
 }

chrome/browser/resource_coordinator/tab_manager.cc

@@ -482,7 +482,13 @@ void TabManager::PurgeBackgroundedTabsIfNeeded() {
     DCHECK(tab_lifecycle_unit_external);
     content::WebContents* content =
         tab_lifecycle_unit_external->GetWebContents();
-    DCHECK(content);
+    // TODO(fdoray): Check if TabLifecycleUnitSource should override
+    // WebContentsObserver::WebContentsDestroyed() as in some situations a
+    // WebContents might get destroyed without a call to
+    // TabStripModelObserver::TabClosingAt, in this case we'll have a
+    // TabLifecycleUnitExternal that points to a null WebContents.
+    if (content == nullptr)
+      return;
 
     content::RenderProcessHost* render_process_host =
         content->GetMainFrame()->GetProcess();