Add more comparators to Optional.h.

BUG=angleproject:1387

Change-Id: If3fc67f99716b2e5e6a8b0ffd139a07a06cdcab8
Reviewed-on: https://chromium-review.googlesource.com/648050
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
diff --git a/src/common/Optional.h b/src/common/Optional.h
index 992a76e..822de4d 100644
--- a/src/common/Optional.h
+++ b/src/common/Optional.h
@@ -56,6 +56,10 @@
 
     bool operator!=(const Optional &other) const { return !(*this == other); }
 
+    bool operator==(const T &value) const { return mValid && (mValue == value); }
+
+    bool operator!=(const T &value) const { return !(*this == value); }
+
   private:
     bool mValid;
     T mValue;