Previously we would always reference mBody in several intermediate
output methods. This made using these traversals from within the
header, or for utility methods, very difficult. Instead, use a stack
where we write to the top InfoLog, and can push/pop from the stack.
This gives us more flexibility.

BUG=angle:878

Change-Id: I8a6c0382bad18b44d75158274c701db13d4d4e65
Reviewed-on: https://chromium-review.googlesource.com/243580Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>