[GN] Allow spaces in output names and other substitutions.

BUG=546894

Review URL: https://codereview.chromium.org/1420043004

Cr-Original-Commit-Position: refs/heads/master@{#356545}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 987f4a5507faeaa0c35c6e550fa993c288dbdfb0