Use new downcast helper for blink::HTMLMeterElement

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

Bug: 891908
Change-Id: Ibd4d7866992b0ead69045e10af68d5faa4f36d0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1794365
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#695566}
4 files changed