[pdf] Speculatively fix PDFIFrameNavigationThrottle crash on Android

This is a speculative fix. But I think it has few / no side effects.

Not 100% sure what's causing this crash, but I have two theories:
 1. Perhaps navigation_handle()->GetWebContents() can return nullptr
    now. On Android only maybe?
 2. I think posting to the UI thread is wrong in any case, we should be
    posting to the current sequence, as otherwise dereferencing the
    weak pointer is invalid. (It's not sequence-safe).

Bug: 1058303
Change-Id: I75fcb86d58602d26e7b8d6991fd933b70881bd99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090300
Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747441}

chrome/browser/plugins/pdf_iframe_navigation_throttle.cc

@@ -8,13 +8,10 @@
 
 #include "base/feature_list.h"
 #include "base/memory/weak_ptr.h"
-#include "base/no_destructor.h"
 #include "base/task/post_task.h"
+#include "base/threading/sequenced_task_runner_handle.h"
 #include "chrome/common/chrome_features.h"
 #include "chrome/common/pdf_util.h"
-#include "content/public/browser/browser_context.h"
-#include "content/public/browser/browser_task_traits.h"
-#include "content/public/browser/browser_thread.h"
 #include "content/public/browser/download_utils.h"
 #include "content/public/browser/navigation_handle.h"
 #include "content/public/browser/render_frame_host.h"
@@ -169,11 +166,14 @@ void PDFIFrameNavigationThrottle::LoadPlaceholderHTML() {
   // an antipattern. Use a helper object scoped to the WebContents lifetime to
   // scope the navigation task to the WebContents lifetime.
   content::WebContents* web_contents = navigation_handle()->GetWebContents();
+  if (!web_contents)
+    return;
+
   PdfWebContentsLifetimeHelper::CreateForWebContents(web_contents);
   PdfWebContentsLifetimeHelper* helper =
       PdfWebContentsLifetimeHelper::FromWebContents(web_contents);
-  base::PostTask(
-      FROM_HERE, {content::BrowserThread::UI},
+  base::SequencedTaskRunnerHandle::Get()->PostTask(
+      FROM_HERE,
       base::BindOnce(&PdfWebContentsLifetimeHelper::NavigateIFrameToPlaceholder,
                      helper->GetWeakPtr(), std::move(params)));
 }