cc: Optimize picture layer tiling raster tile iterator.

cc: Optimize picture layer tiling raster tile iterator.

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

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=285542

[vmpstr, 2014-07-24 00:34:34]

PTAL

[reveman, 2014-07-24 13:09:23]

lg, I just like to see if we can't make some of the code modified by this patch
a bit cleaner before landing this.

https://codereview.chromium.org/411183002/diff/1/cc/resources/picture_layer_t...
File cc/resources/picture_layer_tiling.cc (right):

https://codereview.chromium.org/411183002/diff/1/cc/resources/picture_layer_t...
cc/resources/picture_layer_tiling.cc:918: }
This actually made the code a bit easier for me to understand but I think it
could be made a bit cleaner. Would it make sense to add an enum for the phases
that this iterator goes through? So you could just "phase + 1" to advance and
use a switch case instead of all these elseifs. The enum names might be enough
to describe what is happening here and the comments I asked for in the other
patch might not even be needed.

[vmpstr, 2014-07-24 16:44:13]

I've rebased this on the other patches, ptal.

[reveman, 2014-07-24 16:52:59]

lgtm

[vmpstr, 2014-07-25 02:32:39]

The CQ bit was checked by vmpstr@chromium.org

[commit-bot: I haz the power, 2014-07-25 02:35:31]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/vmpstr@chromium.org/411183002/20001

[commit-bot: I haz the power, 2014-07-25 09:29:06]

Change committed as 285542