GN: Add optional default_args variable to .gn.

Allows a project to specify default overrides for declared arguments.
This is useful for subprojects which might declare arguments with default values
that need to be changed for this project.

BUG=588513

Review-Url: https://codereview.chromium.org/2581193003
Cr-Commit-Position: refs/heads/master@{#445259}
3 files changed