Provide an option to bench drawing individual tiles in bench_pictures.

Provides output like the following:
running bench [1236 12045] androidpolice.skp
  tile_256x256: tile [0,0] out of [5,48]: msecs =   1.00
  tile_256x256: tile [1,0] out of [5,48]: msecs =   1.50
  tile_256x256: tile [2,0] out of [5,48]: msecs =   1.00
  tile_256x256: tile [3,0] out of [5,48]: msecs =   1.50
  tile_256x256: tile [4,0] out of [5,48]: msecs =   2.50
  tile_256x256: tile [0,1] out of [5,48]: msecs =   2.00
  tile_256x256: tile [1,1] out of [5,48]: msecs =   3.50
  tile_256x256: tile [2,1] out of [5,48]: msecs =   3.50
  tile_256x256: tile [3,1] out of [5,48]: msecs =   6.00
  tile_256x256: tile [4,1] out of [5,48]: msecs =   2.50
  tile_256x256: tile [0,2] out of [5,48]: msecs =   2.00

BUG=https://code.google.com/p/skia/issues/detail?id=1016

Review URL: https://codereview.appspot.com/6937047

git-svn-id: http://skia.googlecode.com/svn/trunk@6805 2bbb7eff-a529-9590-31e7-b0007b416f81
5 files changed