Change visibility semantics for Instant.

Change visibility semantics for Instant.

Don't change the visibility of the active tab as the Instant preview
shows or hides. Instant previews should be considered transient a la the
omnibox drop down, and shouldn't affect the active tab's visibility
state. Also, now that we have SetInstantPreviewHeight(), this is a more
reasonable approach when the preview doesn't completely obscure the tab.

Also:

+ Remove the Hide() call when deleting the loader. This was only used in
  the past when there was a fade, and is not needed anymore. Instead,
  reset the state variables correctly.

+ Remove WasShown/WasHidden calls from BrowserInstantController. These
  are instead handled by the view code.

+ The "GetBackingStore called while hidden" DCHECK doesn't fire anymore,
  so enable a test on Windows.

BUG=101593, 101922
R=sky@chromium.org,shishir@chromium.org
TEST=Instant works as usual. The preview gets visibility change events
as usual, but the underlying tab never gets a hidden event.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=158613

[sreeram, 2012-09-21 22:40:33]

Please review.

[sreeram, 2012-09-25 16:00:37]

On 2012/09/21 22:40:33, sreeram wrote:
> Please review.

@sky, ping?

[sky, 2012-09-25 16:21:09]

LGTM

[commit-bot: I haz the power, 2012-09-25 16:22:25]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/sreeram@chromium.org/10959049/10001

[commit-bot: I haz the power, 2012-09-25 18:21:22]

Change committed as 158613