Fix a crash due to null BrowserPluginDelegate

The HeadlessContentRendererClient does not implement CreateBrowserPluginDelegate
which causes a renderer crash in the headless mode for objects which set their
type to "application/browser-plugin". This CL adds a null check to
RenderFrameImpl to stop the crash.

Bug: 958678
Change-Id: Ic90ecbded86e764fb81eb9796b5d9e2732966fb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1598323Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657570}

content/renderer/render_frame_impl.cc

@@ -3934,6 +3934,8 @@ blink::WebPlugin* RenderFrameImpl::CreatePlugin(
     BrowserPluginDelegate* delegate =
         GetContentClient()->renderer()->CreateBrowserPluginDelegate(
             this, WebPluginInfo(), kBrowserPluginMimeType, GURL(params.url));
+    if (!delegate)
+      return nullptr;
     return BrowserPluginManager::Get()->CreateBrowserPlugin(
         this, delegate->GetWeakPtr());
   }