Return HALF_FLOAT as an implementation read type in ES3.

BUG=765953

Change-Id: I4dc79921766975cd75c489887b7e57ec4666fbbb
Reviewed-on: https://chromium-review.googlesource.com/685897
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/renderer/null/FramebufferNULL.cpp b/src/libANGLE/renderer/null/FramebufferNULL.cpp
index 0112623..39a7adc 100644
--- a/src/libANGLE/renderer/null/FramebufferNULL.cpp
+++ b/src/libANGLE/renderer/null/FramebufferNULL.cpp
@@ -109,7 +109,7 @@
 
     const gl::Format &format = readAttachment->getFormat();
     ASSERT(format.info != nullptr);
-    return format.info->getReadPixelsType();
+    return format.info->getReadPixelsType(context->getClientVersion());
 }
 
 gl::Error FramebufferNULL::readPixels(const gl::Context *context,