Use baseVertex as the parameter of drawLineLoop and drawTriangleFan. The
old name is not clear for its usage.

BUG=angleproject:1724

Change-Id: I529a157f055eba9d62952093f37eabb8398697db
Reviewed-on: https://chromium-review.googlesource.com/430255
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>