Adding //third_party/abseil-cpp.

This CL adds Abseil to the Chromium third_party folder. Since it is
a new third_party, the following steps have been taken into account:

 - Add OWNERS: done.
 - Add README.chromium: done.
 - Add a LICENSE file and run related checks: done.
 - Modify Chromium DEPS: Added "-absl" to disable the inclusion of
   Abseil headers from Chromium code.
 - Setting up .gitignore: not necessary.
 - Get a Review: Security review done, Chrome Eng Review done.
 - Ask for a git mirror: not necessary.

In order to avoid to start depend on Abseil, all the build target under
//third_party/abseil-cpp are only visible to //third_party/webrtc and
the main DEPS file prevents Chromium from #including Abseil headers.

Document with the analysis:
https://docs.google.com/document/d/1DgS1-A3rzboTLjpf4m1sqkJgWjnY_Ru2dokk1X1vBDU

Bug: webrtc:8821
Change-Id: I339680e4b5b03b5bbcad82b0a2934b8bd302f916
Reviewed-on: https://chromium-review.googlesource.com/925605
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Patrik Höglund <phoglund@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550114}
diff --git a/DEPS b/DEPS
index 84a00a5..8adce66 100644
--- a/DEPS
+++ b/DEPS
@@ -1220,6 +1220,9 @@
   '+third_party/icu/source/common/unicode',
   '+third_party/icu/source/i18n/unicode',
   '+url',
+
+  # Chromium cannot directly depend on Abseil.
+  '-absl',
 ]
 
 
@@ -1229,6 +1232,7 @@
   'out',
   'skia',
   'testing',
+  'third_party/abseil-cpp',
   'third_party/breakpad/breakpad',
   'v8',
   'win8',