Some scripts take serialized GN values including strings and lists. These are serialized and escaped according to GN rules. In particular, the string escaping is not possible to duplicate with Python's "ast" module. This simple parser will convert values properly.
Add unit tests for new code, as well as existing code in gn_helpers.
Fix omissions in existing ToGNString code.
BUG=573132,571022
Review URL: https://codereview.chromium.org/1553993002
Cr-Original-Commit-Position: refs/heads/master@{#371000}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9dffb54bc426d84e4a2a1a6fb2989bf53237f5a2