Reduce per-tab memory to 128M maximum, reduce default maximum across all tabs to 256M

Reduce per-tab memory to 128M maximum, reduce default maximum across all tabs to 256M.

The more appropriate fix is that we should be more aggressively discarding offscreen tiles, but this will temporarily fix some OOM issues that we're encountering on low-FB parts.

BUG=141377
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=151778

[Ken Russell (switch to Gerrit), 2012-08-15 01:15:24]

The code LGTM but this needs thorough manual testing.

[ccameron, 2012-08-15 18:36:18]

On 2012/08/15 01:15:24, kbr wrote:
> The code LGTM but this needs thorough manual testing.

I've tested this a fair amount locally, and had no unexpected issues.  The
ultimate effect of this code is that the amount of memory given to the
compositor for a given amount vidmem given to the foreground windows, as a
function of the number of foreground windows is changed.

After this change it is
1 window:  128MB each
2 windows: 128MB each
3 windows:  86MB each
4+ windows: 64MB each

Before this change it was
1 window:  448MB each
2 windows: 224MB each
3 windows: 150MB each
4 windows: 112MB each
5 windows:  89MB each
6 windows:  75MB each
7+ windows: 64MB each

Every page I went to had no issues rendering with 86MB.  The CSS volumetric
renderer dropped some tiles at 64MB (so this will happen at 4 windows open as
opposed to 7).

[nduca, 2012-08-15 18:42:28]

Remind me what happens content goes over? We dont draw the tiles?

[ccameron, 2012-08-15 18:45:33]

On 2012/08/15 18:42:28, nduca wrote:
> Remind me what happens content goes over? We dont draw the tiles?

Exactly.  At fullscreen-and-64MB, the 3D CSS demo has some text missing from it
("Hackery, Math, and Design", in particular).  At 32MB (which we don't get to),
part of the brain is missing.

[nduca, 2012-08-15 18:49:26]

(LGTm fwiw, just trying to remind myself what happens when we get this wrong.)

[commit-bot: I haz the power, 2012-08-15 21:12:14]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ccameron@chromium.org/10827342/2001

[commit-bot: I haz the power, 2012-08-15 23:06:42]

Change committed as 151778