tools/gn: don't allow trailing junk or non-literals in "value" conversion

See also discussion on gn-dev:
https://groups.google.com/a/chromium.org/d/msg/gn-dev/hHRC1HuWBo8/drOd5dTvO5oJ

Review URL: https://codereview.chromium.org/1048913003

Cr-Original-Commit-Position: refs/heads/master@{#323146}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 5557a417ceec6573401493c0760ff914e75795a8
4 files changed
tree: 598afcaba5d15a42fb8e87b1d79fe3aef8c54d78
  1. tools/