Add support for module-definition files to gn BUG= Review URL: https://codereview.chromium.org/1160773002 Cr-Original-Commit-Position: refs/heads/master@{#332355} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: b73b2de6a23e3220c79c6bb9e9f3a7b4dd20d2dd