Pepper: Ensure weak pointer is still valid before use.

Bug: 1146675
Change-Id: I382dcb5c0b09a26e3c397ebef46947f626e2aef9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2527065Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825558}

content/browser/renderer_host/pepper/pepper_file_io_host.cc

@@ -247,7 +247,12 @@ void PepperFileIOHost::GotUIThreadStuffForInternalFileSystems(
     return;
   }
 
-  DCHECK(file_system_host_.get());
+  if (!file_system_host_.get()) {
+    reply_context.params.set_result(PP_ERROR_FAILED);
+    SendOpenErrorReply(reply_context);
+    return;
+  }
+
   DCHECK(file_system_host_->GetFileSystemOperationRunner());
 
   file_system_host_->GetFileSystemOperationRunner()->OpenFile(