Use base level to get internal format in GenerateMipMap BUG=605775 Change-Id: I11039ee9ba21dbe2716eed0d3a12a0fb2cf50ea4 Reviewed-on: https://chromium-review.googlesource.com/340116 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>