webrtc-internals: calculate framesReceived - framesDecoded - framesDropped

since the currently calculated metric always includes any frames dropped
before decoding.

Also extend the difference calculator to take more than a single
argument to subtract and do explicit conversion of strings into numbers instead of relying on Javascript semantics for that.

BUG=None

Change-Id: If13bde153b9ee22fc43594817840169bc0ddf924
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3655462
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1005790}