Fix fast/block/float/4145535Crash.html with scheduler change

We wish to prioritize loading and compositing tasks until first contentful
paint. This changes the ordering of
LocalFrameView::UpdatePluginsTimerFired  which breaks (or makes flaky)
some web_tests, typically the following console log goes missing:

  CONSOLE MESSAGE: Blink Test Plugin: initializing

This patch fixes this by ensuring LocalFrameView::UpdatePluginsTimerFired
runs in FIFO order with loading tasks.

The spec doesn't seem to specify which task source should be used so I
believe we're free to make this change.

Bug: 971191
Change-Id: Ib62fafd7cf1f86835a7c5b749fa0639f884e7089
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1795790Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696894}

third_party/blink/renderer/core/frame/local_frame_view.cc

@@ -242,7 +242,8 @@ LocalFrameView::LocalFrameView(LocalFrame& frame, IntRect frame_rect)
       layout_count_for_testing_(0),
       lifecycle_update_count_for_testing_(0),
       nested_layout_count_(0),
-      update_plugins_timer_(frame.GetTaskRunner(TaskType::kInternalDefault),
+      // We want plugin updates to happen in FIFO order with loading tasks.
+      update_plugins_timer_(frame.GetTaskRunner(TaskType::kInternalLoading),
                             this,
                             &LocalFrameView::UpdatePluginsTimerFired),
       first_layout_(true),