GitLab

As one of the first steps in making the build work with Python 3,
we will need a way to indicate which actions still require Python 2
to run (most work fine under Python 3 already, but some do not).

This CL introduces a python2_action() template, which works just
like a regular action() rule, but ensures that the script runs
under Python 2 instead of whichever Python version you happen to be
running.

This CL also switches //tools/metrics:histograms_xml to use it
(because it seems to need it) as a proof that it works correctly
and does not regress in a normal (python 2) build.

Bug: 941669, 1118214
Change-Id: I98fbc073cf9443acece3caaae2ed35f9d60c38e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363655
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799909}