Add flags to shader translator to emulate gl_DrawID

Adds support for translating gl_DrawID for implementation of
GL_ANGLE_multi_draw.

Currently the change only supports and allows emulation of the
draw id using a uniform variable named `gl_DrawID`. This uniform is
mapped in the translated shader to a hashed name that does not
use the gl_ namespace

Bug: chromium:890539

Change-Id: I08a246ca911e88e733ccdf22f1ed69dcae948e05
Reviewed-on: https://chromium-review.googlesource.com/c/1271957
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
diff --git a/src/libANGLE/Context.cpp b/src/libANGLE/Context.cpp
index 605a3bf..d16fc1c 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -3264,6 +3264,7 @@
     supportedExtensions.bindGeneratesResource = true;
     supportedExtensions.clientArrays          = true;
     supportedExtensions.requestExtension      = true;
+    supportedExtensions.multiDraw             = true;
 
     // Enable the no error extension if the context was created with the flag.
     supportedExtensions.noError = mSkipValidation;