components/gc: Hello world

This sets up a directory for a C++ garbage collection library. The
library will use the core of Oilpan (currently sitting in
third_party/blink/renderer/heap) without depending on WTF or Blink
internals.

Design doc: http://bit.ly/oilpan-library-proposal

"Would you tell me, please, which way I ought to go from here?"
"That depends a good deal on where you want to get to," said the Cat.
"I don't much care where--" said Alice.
"Then it doesn't matter which way you go," said the Cat.
"--so long as I get SOMEWHERE," Alice added as an explanation.
"Oh, you're sure to do that," said the Cat, "if you only walk long
enough."
        -- Lewis Carroll

Bug: 1056170
Change-Id: I231f596783d742800b53730af6dcb84b11ffe389
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2077992
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745173}

components/gc/OWNERS

+bikineev@chromium.org
+haraken@chromium.org
+mlippautz@chromium.org
+omerkatz@chromium.org
+
+# TEAM: oilpan-reviews@chromium.org
+# COMPONENT: Blink>MemoryAllocator>GarbageCollection

components/gc/README.md

+# Oilpan - C++ Garbage Collection
+
+Oilpan is an open-source garbage collection library for C++ in Chromium.
+
+The library is currently under construction. The Blink-specific parts can be found in third_party/blink/renderer/platform/heap.