Push API: Clarify needed_but_not_shown logic

Push API: Clarify needed_but_not_shown logic

The final if condition in
PushMessagingServiceImpl::DidGetNotificationsShown is always true.

This patch replaces the if with DCHECKs to make the code more
understandable.

Committed: https://crrev.com/c8d41cd2e5c44a43e1606555bfe8b7bfdf376cb9
Cr-Commit-Position: refs/heads/master@{#326811}

[johnme, 2015-04-23 16:13:53]

johnme@chromium.org changed reviewers:
+ mvanouwerkerk@chromium.org

[Michael van Ouwerkerk, 2015-04-24 12:35:17]

lgtm

[johnme, 2015-04-24 15:21:22]

The CQ bit was checked by johnme@chromium.org

[commit-bot: I haz the power, 2015-04-24 15:22:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1086393005/1

[commit-bot: I haz the power, 2015-04-24 16:14:13]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-24 16:14:54]

Patchset 1 (id:??) landed as
https://crrev.com/c8d41cd2e5c44a43e1606555bfe8b7bfdf376cb9
Cr-Commit-Position: refs/heads/master@{#326811}

[Peter Beverloo, 2015-04-26 19:42:22]

peter@chromium.org changed reviewers:
+ peter@chromium.org

[Peter Beverloo, 2015-04-26 19:42:22]

https://codereview.chromium.org/1086393005/diff/1/chrome/browser/push_messagi...
File chrome/browser/push_messaging/push_messaging_service_impl.cc (right):

https://codereview.chromium.org/1086393005/diff/1/chrome/browser/push_messagi...
chrome/browser/push_messaging/push_messaging_service_impl.cc:386:
DCHECK(notification_shown || notification_needed);  // Caller must ensure this
nit: I prefer to have DCHECKs validating the arguments as the very first thing
in a method, as they're essentially "entry conditions".

I don't think "Caller must ensure this" adds much value - there would not be a
DCHECK if this were not the case.