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This patch ensures that we only create tiling data iterators when
we visit the corresponding region.

Relevant perf data from a macbook:

Before:
tiling_raster_tile_iterator_construct:
0_0_100x100= 5828515 runs/s
50_0_100x100= 5530272 runs/s
100_0_100x100= 2730390 runs/s
150_0_100x100= 3735413.25 runs/s

tiling_raster_tile_iterator_construct_and_iterate:
32_100x100= 576050 runs/s
32_500x500= 601867.5625 runs/s
64_100x100= 353348.21875 runs/s
64_500x500= 362777.46875 runs/s

After:
tiling_raster_tile_iterator_construct:
0_0_100x100= 7913765 runs/s (+36%)
50_0_100x100= 7875735 runs/s (+42%)
100_0_100x100= 3382876.5 runs/s (+23%)
150_0_100x100= 5191805 runs/s (+39%)

tiling_raster_tile_iterator_construct_and_iterate:
32_100x100= 599836.6875 runs/s (+4%)
32_500x500= 629638.75 runs/s (+5%)
64_100x100= 357503.9375 runs/s (+<1%)
64_500x500= 372805.71875 runs/s (+3%)

R=reveman

Review URL: https://codereview.chromium.org/411183002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285542 0039d316-1c4b-4281-b951-d872f2087c98