Use new downcast helper for blink::HTMLTableRowElement

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

Bug: 891908
Change-Id: I4095c5486fbdf63c3c692b295f23b18d626ab48d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899640
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#713505}
7 files changed