Force Python 3 to be used in build.

This CL makes Python 3 be the default script executable used by Ninja
to run action() and action_foreach()'s during the build.

This does *not* mean that everything works with Python3; many targets
may still use Python 2 via the python2_action() and
python2_action_foreach() targets or via custom logic.

However, it does mean that it should prevent regressions from
creeping in.

This CL should not be taken to imply that we can start to write
Python3-specific code that can run during the build; we should
still be trying to ensure that everything is compatible with Python2.

Also, this CL does not enable the use of vpython in actions; we should
still avoid that because vpython is slower than direct Python
invocations.

Bug: 1112471
Change-Id: I472b032b500715156d955b0758640a7ed0384305
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515787Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#836234}

.gn

@@ -8,6 +8,10 @@ import("//third_party/angle/dotfile_settings.gni")
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
+# The python interpreter to use by default. On Windows, this will look
+# for python3.exe and python3.bat.
+script_executable = "python3"
+
 # These arguments override the default values for items in a declare_args
 # block. "gn args" in turn can override these.
 #