Clean up unnecessary Create() factory functions in modules' medaistream and service_worker

As advised in [1], this CL removes unnecessary Foo::Create() in
//third_party/blink/renderer/modules/mediastream and service_worker and make
their constructors public, then make the callers use std::make_unique<Foo>
instead of their Create() factory functions.

[1] https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/iJ1bawbxbWs/vEdfT5QtBgAJ

Bug: 939691
Change-Id: If0e295dcc02093c7c70d29e559705e004ed930cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525260
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#641158}
6 files changed