"Refactor BufferStorage11" caused a performance regression by dropping the line
which sets mReadUsageCount to 0. This causes markBufferUsage() to clear mResovedData
which then has to be restored on the next getData() call.

BUG=angle:613

Change-Id: I61c4b4960305a69b315c20ced66265a770002338
Reviewed-on: https://chromium-review.googlesource.com/197503Tested-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>