Blink's platform/graphics to use default LSI

Previously, ParentLocalSurfaceIdAllocator would default to an invalid
LocalSurfaceId. A new LSI had to be explicitly allocated before the
normal behavior could use it.

With ParentLocalSurfaceIdAllocator now default initializing a valid LSI,
the various call sites no longer need to explicitly check for the
uninitialized state.

Blink's platform/graphics have some of those call sites.

BUG=837030

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ie8ec4f6d09e357e9433e583e0a79abb72de73385
Reviewed-on: https://chromium-review.googlesource.com/1038140Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555243}

third_party/blink/renderer/platform/graphics/offscreen_canvas_frame_dispatcher.cc

@@ -47,7 +47,8 @@ OffscreenCanvasFrameDispatcher::OffscreenCanvasFrameDispatcher(
   if (frame_sink_id_.is_valid()) {
     // Only frameless canvas pass an invalid frame sink id; we don't create
     // mojo channel for this special case.
-    current_local_surface_id_ = parent_local_surface_id_allocator_.GenerateId();
+    current_local_surface_id_ =
+        parent_local_surface_id_allocator_.GetCurrentLocalSurfaceId();
     DCHECK(!sink_.is_bound());
     mojom::blink::OffscreenCanvasProviderPtr provider;
     Platform::Current()->GetInterfaceProvider()->GetInterface(

third_party/blink/renderer/platform/graphics/video_frame_submitter.cc

@@ -25,7 +25,8 @@ VideoFrameSubmitter::VideoFrameSubmitter(
       resource_provider_(std::move(resource_provider)),
       is_rendering_(false),
       weak_ptr_factory_(this) {
-  current_local_surface_id_ = parent_local_surface_id_allocator_.GenerateId();
+  current_local_surface_id_ =
+      parent_local_surface_id_allocator_.GetCurrentLocalSurfaceId();
   DETACH_FROM_THREAD(media_thread_checker_);
 }