The patch addresses code duplication in cases in which main()'s body node
has to be retrieved from the AST.

TEST=angle_unittests

Change-Id: I0ccee41821a5ecb2cdf09f5a928b178d6c037587
Reviewed-on: https://chromium-review.googlesource.com/561521Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>