GN: Fix bootstrap.py
GN: Fix bootstrap.py

//base/thread_task_runner_handle.cc was moved into //base/threading/.

Also for the build to succeed it now needs //base/files/important_file_writer.cc as well.

R=brettw@chromium.org

Review-Url: https://codereview.chromium.org/1972093002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#394528}

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