Merging ANGLE from Upstream

There are some important things to do when pulling in a new revision of ANGLE.

1. Add a note explaining that we are explicitly choosing the non-GPL license for distributing sources based on Bison 2.4+. Add the following text: "* Apple Note: For the avoidance of doubt, Apple elects to distribute this file under the terms of the BSD license." to the files:
   • glslang_tab.cpp
   • glslang_tab.h
   • ExpressionParser.cpp
2. Update ANGLE.plist with the details of the version of ANGLE you are committing.
3. Add preprocessor comments to allow ANGLE to build on older versions of clang. Basically, copy ​this commit (although we no longer run the bison generation step, so you only need to touch the .cpp files)
4. Update ShaderLang.h to include "khrplatform.h" not "KHR/khrplatform.h"
5. Define YY_NO_INPUT in Tokenizer.cpp
6. Copy the change from ​this commit.
7. Then whatever needs to be done to get it to compile on all ports (mostly avoiding clang warnings)

Fixing ANGLE Bugs

When fixing bugs in ANGLE, please create a new bug on Monorail (Google's bug tracker) and attach the patch applied to WebKit so that changes can eventually be merged upstream instead of maintained locally.

​https://bugs.chromium.org/p/angleproject/issues/list