Traversers used during parsing can be vulnerable to stack overflow
since the AST has not yet been validated for max depth. Make sure to
check for traversal depth in traversers used during parsing.

We set the maximum traversal depth in ValidateGlobalInitializer and
ValidateSwitchStatementList to 256, which matches the default value
for validating general AST complexity. The depth check is on
regardless of compiler options. In case the traversers go over the
maximum traversal depth, they fail validation.

BUG=angleproject:2453
TEST=angle_unittests

Change-Id: I89ba576e8ef69663ba35d7b9050a6da319f1757c
Reviewed-on: https://chromium-review.googlesource.com/995795Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>