Eliminated remaining uses of [LegacyInterfaceTypeChecking]

Removed [LegacyInterfaceTypeChecking] from the two remaining idls,
RTCPeerConnection-getStats and FormData. Updated the relevant tests to
match the new behavior.

Seeing as how [LegacyInterfaceTypeChecking] is no longer being used,
I removed the extended attribute from IDLExtendedAttributes.md and
IDLExtendedAttributes.txt.

Bug: 561338
Change-Id: I4783348f485932f08c86ddb58efb6cb713fd5fe4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1507237
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Allison Pastewka <alpastew@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#642126}
22 files changed