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GN writes a Ninja command to re-invoke GN when a build file changes. Previously
this hardcoded the build directory. But if the user renames the build
directory, Ninja will think the files are out of date but GN will keep
regenerating files in another directory.

This changes the directory to "." because Ninja will invoke all commands with
the build directory as the current one.

BUG=639965

Review-Url: https://codereview.chromium.org/2389133003
Cr-Commit-Position: refs/heads/master@{#422995}