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Re-land includes missing headers directory.

This replaces several grouped dependencies with one meta-repo.
The meta-repo will allow us to group multiple inter-dependent
deps rolls into one single roll. This solves the problem of
separate rollers breaking because of the inter-dependencies.
It also allows ANGLE and Chrome to share common bits like the
README and .gitignore files.

Bug: angleproject:5390
Bug: 1153258
Change-Id: I22cefb1bfeeec656c2dbb173701a87a6db9cb8ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593888
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Auto-Submit: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837732}