Do not allow fullscreening during a drag

Do not allow a page to attempt to enter fullscreen while it is being
actively dragged. Things break.

(cherry picked from commit 26594d7)

Fixed: 1315080
Change-Id: Ia175339807284a3e0edddfbf8fb26a4b67fdaf04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3601249
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#995196}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3601574
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/branch-heads/5005@{#138}
Cr-Branched-From: 5b4d945-refs/heads/main@{#992738}

chrome/browser/ui/browser.cc

@@ -1916,6 +1916,11 @@ void Browser::EnumerateDirectory(
 bool Browser::CanEnterFullscreenModeForTab(
     content::RenderFrameHost* requesting_frame,
     const blink::mojom::FullscreenOptions& options) {
+  // If the tab strip isn't editable then a drag session is in progress, and it
+  // is not safe to enter fullscreen. https://crbug.com/1315080
+  if (!tab_strip_model_delegate_->IsTabStripEditable())
+    return false;
+
   return exclusive_access_manager_->fullscreen_controller()
       ->CanEnterFullscreenModeForTab(requesting_frame, options.display_id);
 }