Advertises the NV_pixel_buffer_object extension

BUG=angle:581

Change-Id: I1e55582e94a36e56ef3e0f53ff9233c1a31408f5
Reviewed-on: https://chromium-review.googlesource.com/198238
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Shannon Woods <shannonwoods@chromium.org>

src/libGLESv2/Context.cpp

@@ -3780,6 +3780,7 @@ void Context::initExtensionString()
 
         if (supportsPBOs())
         {
+            mExtensionStringList.push_back("NV_pixel_buffer_object");
             mExtensionStringList.push_back("GL_OES_mapbuffer");
             mExtensionStringList.push_back("GL_EXT_map_buffer_range");
         }