Reading back buffer data is required for index range validation but
without glMapBufferRange it is not possible to read back buffer data on
OpenGL ES.  To work around this, keep a shadow copy of the buffer data
when this function is not available.

BUG=angleproject:1145

Change-Id: I8e9b3b174574316d3af0022bd29c7d9c96d168c3
Reviewed-on: https://chromium-review.googlesource.com/324092Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>