Do not call glDrawBuffersARB when the extension is absent

      The function Framebuffer::ChangeDrawBuffersHelper was assuming
that binding of an FBO with no GL_COLOR_ATTACHMENT0 was
accompanied by setting the first draw buffer to GL_NONE with
glDrawBuffersARB in the fix introduced in:
https://codereview.chromium.org/315283002

When the extension is not present, it can be assumed that the
glDrawBuffersARB state need not be set and restored.

BUG=(wrangling)
NOTRY=True

Review URL: https://codereview.chromium.org/375413003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282393 0039d316-1c4b-4281-b951-d872f2087c98