[Android] Allow redundant |StartObservingRootWindow()| calls

[Android] Allow redundant |StartObservingRootWindow()| calls

Redundant requests to observe the window are harmless, and should be supported
given the undefined relationship and timing between
|RWHVAndroid::SetContentViewCore()| and |RWHVAndroid::WasShown()|, both of which
can trigger observing of the root window.

BUG=406651, 394604
NOTRY=true
TBR=sievers@chromium.org

Committed: https://crrev.com/789317e700bb3966398e4a0c5ec930bbcee9d0cf
Cr-Commit-Position: refs/heads/master@{#291596}

[jdduke (slow), 2014-08-23 01:13:44]

sievers@: Would you prefer an early return here? I don't think it hurts us to
just fall through (that's what we did previously), either way.

[jdduke (slow), 2014-08-23 01:19:30]

The CQ bit was checked by jdduke@chromium.org

[boliu, 2014-08-23 01:24:50]

I think it should either support repeated calling by early out, or DCHECK and
fix all callsites. This in-between thing is kinda weird and hard to reason
about. Also start and stop should have the same behavior.

cq is down and tree is closed due to the git migration btw, kinda surprised the
try bots are still working

[jdduke (slow), 2014-08-23 01:45:05]

The CQ bit was checked by jdduke@chromium.org

[commit-bot: I haz the power, 2014-08-23 04:56:17]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jdduke@chromium.org/493053004/20001

[commit-bot: I haz the power, 2014-08-23 21:53:22]

Committed patchset #2 (20001) as c1ca38978dc28791af4c7460fbe45c0ee9e49776

[no sievers, 2014-08-25 17:11:18]

lgtm

[commit-bot: I haz the power, 2014-09-10 02:31:38]

Patchset 2 (id:??) landed as
https://crrev.com/789317e700bb3966398e4a0c5ec930bbcee9d0cf
Cr-Commit-Position: refs/heads/master@{#291596}