Use std::stringstream when possible, and use Context's caps instead
of the Renderer's.

BUG=angleproject:754

Change-Id: I2dc773709bbd612ab7ea372a358337c0a81869a3
Reviewed-on: https://chromium-review.googlesource.com/307872Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>