RenderThreadImpl 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.

RenderThreadImpl is one of those call sites.

BUG=837030

Change-Id: Ia1ee9524027d512980f97d08bfc61e93536e3b26
Reviewed-on: https://chromium-review.googlesource.com/1038060Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555258}

content/renderer/render_thread_impl.cc

@@ -419,9 +419,7 @@ class RendererLocalSurfaceIdProvider : public viz::LocalSurfaceIdProvider {
       const viz::CompositorFrame& frame) override {
     auto new_surface_properties =
         RenderWidgetSurfaceProperties::FromCompositorFrame(frame);
-    if (!parent_local_surface_id_allocator_.GetCurrentLocalSurfaceId()
-             .is_valid() ||
-        new_surface_properties != surface_properties_) {
+    if (new_surface_properties != surface_properties_) {
       parent_local_surface_id_allocator_.GenerateId();
       surface_properties_ = new_surface_properties;
     }