Convert media elements to use UA shadow instead of V0 shadow

Found some media control elements accidentally using V0 shadow
root instead of User-Agent shadow root, and this fixes the cases.

To avoid further misuses, I'll rename the API in the following CL
https://chromium-review.googlesource.com/c/chromium/src/+/1057047

Bug: 787717, 776656
Change-Id: I45274ccb399ae57ea7207402323f49efd2188d9c
Reviewed-on: https://chromium-review.googlesource.com/1056939Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Takayoshi Kochi <kochi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558266}

third_party/blink/renderer/modules/media_controls/elements/media_control_loading_panel_element.cc

@@ -32,7 +32,7 @@ MediaControlLoadingPanelElement::MediaControlLoadingPanelElement(
     MediaControlsImpl& media_controls)
     : MediaControlDivElement(media_controls, kMediaControlsPanel) {
   SetShadowPseudoId(AtomicString("-internal-media-controls-loading-panel"));
-  CreateShadowRootInternal();
+  CreateUserAgentShadowRoot();
 
   // The loading panel should always start hidden.
   SetIsWanted(false);

third_party/blink/renderer/modules/media_controls/elements/media_control_scrubbing_message_element.cc

@@ -18,7 +18,7 @@ MediaControlScrubbingMessageElement::MediaControlScrubbingMessageElement(
     MediaControlsImpl& media_controls)
     : MediaControlDivElement(media_controls, kMediaScrubbingMessage) {
   SetShadowPseudoId(AtomicString("-internal-media-controls-scrubbing-message"));
-  CreateShadowRootInternal();
+  CreateUserAgentShadowRoot();
   SetIsWanted(false);
 }