In some cases, when the app would call glBufferSubData to do a small
data update in an existing vertex buffer, the sync code would not
flush out the data to the native D3D11 buffer from the temporary
staging buffer.

Fix this problem by notifying the VertexArray11 class when buffer
data is updated. Note that in the future we can improve this by
using a different update notification for when the buffer data changes
and when the underlying storage changes. For now take a very broad
approach.

BUG=angleproject:1156

Change-Id: I2e0fabc97c1f1d5a14d609247e61c602e9a5a85f
Reviewed-on: https://chromium-review.googlesource.com/644208Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>