Some places would compare with "== GL_TRUE" and others with "!= GL_FALSE".
This behaviour is not in the OpenGL spec but "!= GL_FALSE" is the most
standard and follows the same rules as C and C++.

Remove un-necessary validation that params are either GL_TRUE or
GL_FALSE.

Update some internal storage from GLboolean to bool.

BUG=angleproject:2258

Change-Id: I12adbe2d24318a206521ca6ad1099ee7e2bf677e
Reviewed-on: https://chromium-review.googlesource.com/779799Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>