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Spawning many python processes can be bottleneck of build time.
This patch is to improve the time by replacing action_foreach to action for mojom related actions.

Especially, windows buildbot sometimes uses a few minutes for mojom generator.
https://chromium-build-stats.appspot.com/ninja_log/2018/03/22/win1357-c4/ninja_log.win1357-c4.chrome-bot.20180322-005012.7392.gz/trace.html from https://ci.chromium.org/buildbot/tryserver.chromium.win/win7_chromium_rel_ng/127904
The trace shows there are many mojom related actions around 70s~110s and 190s~260s (Those are emphasized by typing mojom to search box.)

I took build stats after touching mojom_bindings_generator.py
On Z840 Windows 10, time to build content/common:mojo_bindings improved from 16.755s to 7.646s and the number of build target is reduced from 2342 to 600.
On Z840 Linux, time to build content/common:mojo_bindings improved from 7.501s to 4.279s and the number of build target is reduced from 2340 to 611.


Change-Id: I56168a233839e1386f31c0b769b263b43dc865d8
Reviewed-on: https://chromium-review.googlesource.com/974982Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546048}