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r531270 refactored HeaderChecker to use an atomic counter to track the number of
header-checking tasks.  The main thread waits on the task count becoming zero,
and the task counter is decremented whenever a task completes.  However, the
main thread didn't increment the task counter when it created a task, so the
counter would count into negative values and never reach zero, hanging gn
indefinitely.  The fix is to increment the counter when posting a task.

R=dpranke
CC=fdoray

Change-Id: Id55f943cf0e2af3e9e5af5f3dddf8a7b7671eaac
Reviewed-on: https://chromium-review.googlesource.com/889930Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532684}