Set write buffer flags only in SkWriteBuffer and SkFlatController constructors.

Set write buffer flags only in SkWriteBuffer and SkFlatController constructors.

This is a baby step toward refactored (and faster in-process) typeface and flattenable factory encoding and decoding.  The sooner SkWriteBuffer knows its flags, the better.

Next steps will be to rearrange Sk{Read,Write}Buffer members into disjoint strategies to handle typefaces and flattenable factories: one for in-process, one for cross-process, one when validating.

BUG=skia:
R=reed@google.com, scroggo@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/138803005

git-svn-id: http://skia.googlecode.com/svn/trunk@13253 2bbb7eff-a529-9590-31e7-b0007b416f81