BrowserPlugin::Initialize to post a loading task

We want to prioritize loading and compositing tasks until first first
contentful paint, however that introduces some unintentional task
re-ordering which breaks
ChromeSitePerProcessBrowserPluginPDFTest.SendSyntheticTapGestureOOPIF.
Changing BrowserPlugin::Initialize to post a loading task removes
one source of reordering with the new scheduler policy, fixing the
the browser test.

Bug: 971191
Change-Id: I177d9189c980601893223425e36cd6657d058510
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1819254Reviewed-by: James MacLean <wjmaclean@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699737}

content/renderer/browser_plugin/browser_plugin.cc

@@ -91,7 +91,9 @@ BrowserPlugin::BrowserPlugin(
       browser_plugin_instance_id_(browser_plugin::kInstanceIDNone),
       delegate_(delegate),
       task_runner_(
-          render_frame->GetTaskRunner(blink::TaskType::kInternalDefault)) {
+          // Make sure BrowserPlugin::UpdateInternalInstanceId runs in FIFO
+          // order with respect to other loading tasks.
+          render_frame->GetTaskRunner(blink::TaskType::kInternalLoading)) {
   browser_plugin_instance_id_ =
       BrowserPluginManager::Get()->GetNextInstanceID();