(Compared to https://codereview.chromium.org/2308313003/, this version
fixes chrome/installer/linux/BUILD.gn by including all required files)

There is nothing really chrome-specific in the process_version()
template, and if we stop always passing some files in chrome/ on every
invocation it is possible to generalize it and move it to build/util
instead (and fix some layering violation comments along the way).

Compared to its previous incarnation, process_version() no longer passes
LASTCHANGE, BRANDING and VERSION automatically to version.py. This makes
it easier for callers to specify their own |sources| with values that
may override those set in those 3 files.

A new wrapper, process_version_rc_template(), was introduced to cater
for callers (generally Windows ones) who need to process a .rc.version
file that requires the values from LASTCHANGE, BRANDING and VERSION. It
always passes those 3 files to process_version() before any optio...