Remove WebRTCStatsReportCallbackResolver::Create in modules/peerconnection

As advised in [1], this CL removes unnecessary WebRTCStatsReportCallbackResolver::Create()
in //third_party/blink/renderer/modules/indexeddb and make their constructors
public, then make the callers use std::make_unique<WebRTCStatsReportCallbackResolver>
instead of it's Create() factory function.

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

Bug: 939691
Change-Id: I2adebce9cf1df122e400a08a83bc9f9aa895c6b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524391
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#641157}
5 files changed