Rename build/config/mac/gen_plist.py to build/config/mac/plist_util.py
after splitting the possible action (currently variable substitution and
plist merging).

Extend tweak_info_plist template to add another parameter info_plists
corresponding to a list of path to merge before tweaking (this allow for
adding partial plist files depending on configuration options).

Always pass the --platform variable based on the value of $current_os
to the tweak_info_plist template and make args optional.

Refactor info_plist template to use separate steps for merge and variable
substitution (as the script now implement them as two separate actions).

BUG=613543

Review-Url: https://codereview.chromium.org/2480433002
Cr-Commit-Position: refs/heads/master@{#429830}