Generate fewer tiles when zooming in.

Generate fewer tiles when zooming in.

When we zoom in, the page goes from being able to scroll only
vertically to being able to scroll both vertically and horizontally.
This results in, roughly, a squaring of the visible content area
(actually a bit less due to discretization). This patch clamps the
are examined when zooming in. A more principled version would
attempt to explicitly keep the area constant.

BUG=174222
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=181029

[ccameron, 2013-02-06 19:39:54]

This does a pretty good job (drops usage from ~225MB to ~90MB when zoomed in).
Lots of room for tweaking left, though.

[nduca, 2013-02-06 19:51:51]

lgtmnomnomnom