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The main improvement is that source files with unhandled extensions
other than .h, for example .json and .idl, are not counted towards the
merge limit, and thus can't lead to too many chunks being generated.

By restructuring, the code also iterates the source file list once
instead of twice, and calls get_path_info() once per source file instead
of twice.

Change-Id: I130c815c84fff08b6ad2d6f36434140efbeb2d29
Reviewed-on: https://chromium-review.googlesource.com/850397Reviewed-by: Daniel Bratell <bratell@opera.com>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#527099}