ninja: Give WriteSources() and WriteLink() an explicit ninja_file parameter.
ninja: Give WriteSources() and WriteLink() an explicit ninja_file parameter.

For fat binary support on mac, compile and link lines will need different
flags for every arch (i386, x86_64, etc). The only way ninja currently supports
variable scoping is through subninja files, so compile and link commands for
multi-arch compiles will need to go into subninja files. Prepare WriteSources()
and WriteLink() for this.

No functionality change in this CL.

BUG=chromium:90185
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/21930004
old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google