It doesn't need to use a traverser, since all function definitions can
be found simply by iterating over the children of the root node.

BUG=angleproject:2040
TEST=angle_end2end_tests

Change-Id: I380942f90a0e73152f296b98d1fb027762d913b0
Reviewed-on: https://chromium-review.googlesource.com/508689Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>