Fix compile on linux_redux bots

Fix compile on linux_redux bots, caused by http://crrev.com/113808

linux_redux does not have safe_browsing enabled in the GYP variables,
which causes all of chrome/browser/safe_browsing to be excluded from
the chrome_browser target - including safe_browsing_tab_observer.[cc,h]

This enables a stub SafeBrowsingTabObserver to be compiled when
safe_browsing==0, fixing the linker errors from TabContentsWrapper having
a SafeBrowsingTabObserver member.

BUG=none

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=114553

[Ryan Sleevi, 2011-12-14 23:02:52]

sail: rubberstamp?
avi: review || fyi?

[Ryan Sleevi, 2011-12-14 23:06:28]

mattm: chrome/browser/safe_browsing/OWNERS stamp

[sail, 2011-12-14 23:08:39]

lgtm

[Avi (use Gerrit), 2011-12-14 23:13:04]

D'oh. LGTM

[commit-bot: I haz the power, 2011-12-14 23:13:54]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rsleevi@chromium.org/8909017/1

[commit-bot: I haz the power, 2011-12-14 23:34:35]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rsleevi@chromium.org/8909017/6001

[mattm, 2011-12-14 23:52:00]

lgtm 
Maybe mention in the description which cl broke it.

[commit-bot: I haz the power, 2011-12-15 00:53:46]

Commit queue rejected this change because the description was changed
between the time the change entered the commit queue and the time it
was ready to commit. You can safely check the commit box again.