Clean up unnecessary Create() factory functions in platform/bindings

As advised in [1], this CL removes unnecessary Foo::Create() in
//third_party/blink/renderer/platform/bindings 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: I0a28a2b8ce259313b6e2c55ed69d6d52eb36437d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525767
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#641154}
8 files changed