Put BlinkLeakDetector code back to core/

Although BlinkLeakDetector should and will in the future live in controller/, currently it is better for the code to be in core/ to make it easy to add a devtools protocol method.
This CL puts the code back to core/leak_detector.

This CL removes unnecessary lines added in BUILD.gn file in core/ from the original CL ( https://chromium-review.googlesource.com/c/chromium/src/+/622816/1 ), which was reverted.

Bug: 
Change-Id: I973c4b64fea95bba2822d8f20c8a849b9f4fca70
Reviewed-on: https://chromium-review.googlesource.com/624769
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuzu Saijo <yuzus@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496223}
11 files changed