Build libwebrtc unit tests executables

Build libwebrtc unit tests executables
https://bugs.webkit.org/show_bug.cgi?id=177211

Patch by Youenn Fablet <youenn@apple.com> on 2017-10-03
Reviewed by Alex Christensen.

.:

* WebKit.xcworkspace/xcshareddata/xcschemes/All Source.xcscheme:

Source/ThirdParty/libwebrtc:

Adding support for a new target called unittests that will be several executables.
Each executable run unit tests dedicated to a part of libwebrtc.

Adding one target/executable per unit test suite.
Adding one composite target to build all unit test targets.
Adding a target to build a static libwebrtctest library.
The static libwebrtctest library is then linked to each unit test executable which is also linked to libwebrtc dylib.

Some unit tests require a default codec (VP8) that is disabled in libwebrtc.
This ends up making some tests crashing.
An additional work should follow to execute only the meaningful subset of tests.

* Configurations/libwebrtc-base.xcconfig: Added.
* Configurations/libwebrtc-test-static.xcconfig: Added.
* Configurations/rtc_pc_unittests.xcconfig: Added.
* Source/third_party/gflags/gen/posix/include/private/config.h:
* Source/webrtc/modules/audio_coding/neteq/tools/neteq_test.cc: Replacing FATAL by RTC_FATAL.
* Source/webrtc/sdk/objc/Framework/Classes/Common/helpers.mm: Removing UIKit dependency.
* Source/webrtc/test/gmock.h: Using googletest version instead of checking in testing folder.
* Source/webrtc/test/gtest.h: Ditto.
* Source/webrtc/test/rtp_file_reader.cc: Replacing FATAL by RTC_FATAL.
* libwebrtc.xcodeproj/project.pbxproj:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@222775 268f45cc-cd09-0410-ab3c-d52691b4dbfc