Add support for ES31 context creation

The dEQP test for context creation passes.

SH_WEBGL3_SPEC has been added, but it should be considered whether we
should keep it, remove it or rename it. It was added so that there is
a webgl mapping to es 310 shaders. Check Compiler.cpp. The bison file
has been modified so that some tokens from es3 can be also used in
es31 as well.

A separate macro ES3_1_ONLY is added so that some tokens are limited
only for es 310 shaders.

BUG=angleproject:1442
TEST=angle_unittests

Change-Id: I2e5ca227c96046c30dc796ab934f3fda9c533eba
Reviewed-on: https://chromium-review.googlesource.com/360300
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
diff --git a/src/compiler/translator/DirectiveHandler.cpp b/src/compiler/translator/DirectiveHandler.cpp
index ff8a69e..f8081ec 100644
--- a/src/compiler/translator/DirectiveHandler.cpp
+++ b/src/compiler/translator/DirectiveHandler.cpp
@@ -182,8 +182,7 @@
 void TDirectiveHandler::handleVersion(const pp::SourceLocation& loc,
                                       int version)
 {
-    if (version == 100 ||
-        version == 300)
+    if (version == 100 || version == 300 || version == 310)
     {
         mShaderVersion = version;
     }