Fix missing query for GL_NUM_REQUESTABLE_EXTENSIONS_ANGLE.

This enum was validated but not implemented.

TEST=RequestExtensionTest.Queries

BUG=890689

Change-Id: I7285e45cef53b197d61cd0dfa9d2bacb70e23e64
Reviewed-on: https://chromium-review.googlesource.com/1255142
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/Context.cpp b/src/libANGLE/Context.cpp
index 1837199..68fa842 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -1603,6 +1603,11 @@
             *params = static_cast<GLint>(mExtensionStrings.size());
             break;
 
+        // GL_ANGLE_request_extension
+        case GL_NUM_REQUESTABLE_EXTENSIONS_ANGLE:
+            *params = static_cast<GLint>(mRequestableExtensionStrings.size());
+            break;
+
         // GL_KHR_debug
         case GL_MAX_DEBUG_MESSAGE_LENGTH:
             *params = mExtensions.maxDebugMessageLength;