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This is a continuation of tkent's work in
https://chromium-review.googlesource.com/c/chromium/src/+/2397378

The sub-directories of Blink core depend on each other, and we don't
intend to make independent libraries for each of them. We want a
single library containing everything.

Currently we have build targets in each of them, and it's very
difficult to declare their dependencies correctly in BUILD.gn
due to the massive amount of circular dependencies.

The new approach which this CL introduces:
* A core sub-directory doesn't need BUILD.gn in most cases.
* It should have build.gni instead, and build.gni should define
  source lists.
* core/BUILD.gn refers to the lists. All dependencies will also be
  declared here, since there's no way to determine if a sub-directory
  has successfully declared all necessary dependencies (since it's part
  of one big target now).
* Some of core sub-directories apply optimization flags different
  from others. core/BUILD.gn has 'core_hot' target for them.

There are still a handful of sub-directories under renderer/core/ that
have a BUILD.gn file, because they have special build flags or special
targets. We might want to get rid of those eventually as well.

typed_arrays/BUILD.gn is kept because it has no difficult dependencies,
so it deserves to be a target on its own.

Bug: 800764
Change-Id: I92cd5bac949fe4c541f21131309cfc66743edb26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463919Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818418}