Fix crash in RequestAssistantStructureForActiveBrowserWindow

This crash has been happening quite frequently since this summer
(see crbug.com/1128031).

The crash is at line 41:
  assistant_extra->url = web_contents->GetLastCommittedURL();
Given that we just created |assistant_extra| the line above, it must be
that |web_contents| is invalid.

|web_contents| is created at line 94:
  content::WebContents* web_contents =
      browser->tab_strip_model()->GetActiveWebContents();

It is surprising that this can be nullptr because we check at line 52
to ensure the browser is active (so you'd assume it has active web
content).
But irregardless, I added a null check after line 94.

Bug: 1128031
Test: N/A
Change-Id: Iaabdedf5bb133d608db75a16e96907cc18a0d960
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601276
Commit-Queue: Jeroen Dhollander <jeroendh@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839458}

chrome/browser/ui/ash/assistant/assistant_context_util.cc

@@ -44,6 +44,19 @@ ax::mojom::AssistantExtraPtr CreateAssistantExtra(
   return assistant_extra;
 }
 
+gfx::Rect GetWindowBoundsInPixels(const Browser* browser) {
+  gfx::Rect bounds = browser->window()->GetBounds();
+  gfx::Point top_left = bounds.origin();
+  gfx::Point bottom_right = bounds.bottom_right();
+  const aura::Window* window = browser->window()->GetNativeWindow();
+  const auto* window_tree_host = window->GetRootWindow()->GetHost();
+  // TODO: Revisit once multi-monitor support is planned.
+  window_tree_host->ConvertDIPToScreenInPixels(&top_left);
+  window_tree_host->ConvertDIPToScreenInPixels(&bottom_right);
+  return gfx::Rect(top_left.x(), top_left.y(), bottom_right.x() - top_left.x(),
+                   bottom_right.y() - top_left.y());
+}
+
 }  // namespace
 
 void RequestAssistantStructureForActiveBrowserWindow(
@@ -77,26 +90,22 @@ void RequestAssistantStructureForActiveBrowserWindow(
     return;
   }
 
+  content::WebContents* web_contents =
+      browser->tab_strip_model()->GetActiveWebContents();
+  if (!web_contents) {
+    std::move(callback).Run(nullptr, nullptr);
+    return;
+  }
+
   // We follow same convention as Clank and thus the contents are all in
   // pixels. The bounds of the window need to be converted to pixel in order
   // to be consistent with rest of the view hierarchy.
-  gfx::Rect bounds = browser->window()->GetBounds();
-  gfx::Point top_left = bounds.origin();
-  gfx::Point bottom_right = bounds.bottom_right();
-  auto* window_tree_host = window->GetRootWindow()->GetHost();
-  // TODO: Revisit once multi-monitor support is planned.
-  window_tree_host->ConvertDIPToScreenInPixels(&top_left);
-  window_tree_host->ConvertDIPToScreenInPixels(&bottom_right);
+  gfx::Rect window_bounds = GetWindowBoundsInPixels(browser);
 
-  content::WebContents* web_contents =
-      browser->tab_strip_model()->GetActiveWebContents();
   web_contents->RequestAXTreeSnapshot(
-      base::BindOnce(
-          &CreateAssistantStructureAndRunCallback, std::move(callback),
-          CreateAssistantExtra(web_contents,
-                               gfx::Rect(top_left.x(), top_left.y(),
-                                         bottom_right.x() - top_left.x(),
-                                         bottom_right.y() - top_left.y()))),
+      base::BindOnce(&CreateAssistantStructureAndRunCallback,
+                     std::move(callback),
+                     CreateAssistantExtra(web_contents, window_bounds)),
       ui::kAXModeComplete);
 }