unify how we handle externally-provided storage in SkWriter32, with the goal

unify how we handle externally-provided storage in SkWriter32, with the goal
of simplifying the logic in reserve() so it can be inlined/accelerated.
Committed: https://code.google.com/p/skia/source/detail?r=6923

[reed1, 2012-12-19 17:51:33]

will get some bench numbers next

[reed1, 2012-12-19 18:28:49]

profiling shows 5-10% is gone by having reserve() inlined. Total times (on my
mac proc) are too noisy to see a difference (which may be a problem in our
test/timing).

[robertphillips, 2012-12-19 19:04:59]

LGTM