Use new downcast helper for blink::HTMLTemplateElement

This CL has two goals,
1. Use To<HTMLTemplateElement> and DynamicTo<HTMLTemplateElement> as
   new downcast helper
2. Use IsA<HTMLTemplateElement>(element) in place of
   IsHTMLTemplateElement(element)

Bug: 891908
Change-Id: I4220214482d1c4a78b3f5ef2cd450fabdf00c823
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1901150
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713742}
3 files changed