Change growth function for SkWriter32

Add setCountExact to SkTDArray to allow external control of array growth.
Use it to allow SkWriter to control the buffer growth pattern.
Change buffer growth pattern to 1.5n+4096

BUG=skia:2125
R=tomhudson@google.com, mtklein@google.com, reed@google.com

Author: iancottrell@google.com

Review URL: https://codereview.chromium.org/150663014

git-svn-id: http://skia.googlecode.com/svn/trunk@13401 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed