This caused there to be too few mip levels in the TextureStorage and it
would be discarded if there was a modification to a lower mip.

Also modified Texture::mipLevels to return the number of mip levels rather
than the index of the smallest mip to avoid confusion.

Fixes WebGL D3D9 conformance failures in:
    mipmap-fbo

BUG=angleproject:550

Change-Id: Idf3f543487232edceb520337331b39dab2bd26ba
Reviewed-on: https://chromium-review.googlesource.com/192341Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>