PSA: gclient syntax and schema validation enabled by default

[Paweł Hajdan, Jr.]

With https://chromium-review.googlesource.com/c/509451/ syntax and validation for gclient DEPS files is enabled by default.

If you see any errors, please pass --disable-syntax-validation to gclient as an escape hatch, and make sure to report them.

If you need to revert above CL, note the revert may also need to be rolled into tools/build repo (see https://chromium-review.googlesource.com/c/512162/).

This is part of the effort to support conditionals and internal dependencies in gclient. Also see https://goto.google.com/fyuix for some internal announcements.

Paweł