[mac] Check if GetAssociatedFrameWidget() is valid before GetStringAtPoint()

This CL checks if GetAssociatedFrameWidget() is valid before
GetStringAtPoint() since |blink_frame_widget_| from RenderWidgetHostImpl
could be unbound at some point. So, like other methods in
RenderWidgetHostImpl, TextInputClientMac::GetStringAtPoint() also check
GetAssociatedFrameWidget() is valid.

Bug: 1136471
Change-Id: Ia5a17e9ebf14bd0e3743b6236ddde0c08f0e4969
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462921Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Julie Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#816043}

content/browser/renderer_host/text_input_client_mac.mm

@@ -65,8 +65,12 @@ void TextInputClientMac::GetStringAtPoint(RenderWidgetHost* rwh,
                                           const gfx::Point& point,
                                           GetStringCallback callback) {
   RenderWidgetHostImpl* rwhi = RenderWidgetHostImpl::From(rwh);
-  rwhi->GetAssociatedFrameWidget()->GetStringAtPoint(point,
-                                                     std::move(callback));
+  if (rwhi && rwhi->GetAssociatedFrameWidget()) {
+    rwhi->GetAssociatedFrameWidget()->GetStringAtPoint(point,
+                                                       std::move(callback));
+  } else {
+    std::move(callback).Run(nullptr, gfx::Point());
+  }
 }
 
 void TextInputClientMac::GetStringFromRange(RenderWidgetHost* rwh,