Remove WebMediaRecorderHandler and update README.md

This is a part of moving media recorder implementation to blink.
This CL removes unused WebMediaRecorderHandler class and exposed
API in Platform, and update README.md file.

Bug: 960665, 919392
Change-Id: I4a356ecbf35e73ab69d82722feec59a753c46b2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1663460
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670432}
7 files changed