Ensure event pages are unloaded when onSuspend calls an API function

Extensions and apps which call API functions with callbacks such as
chrome.storage.local.set() and chrome.storage.local.get() in the
onSuspend event handler prevents the event page from ever being
unloaded.

The API function finishes between ExtensionProcessManager::OnSuspendAck()
and ExtensionProcessManager::CloseLazyBackgroundPageNow(). The destructor
of UIThreadExtensionFunction results in DecrementLazyKeepaliveCount()
being called in between these two functions, which increments
close_sequence_id. This causes CloseLazyBackgroundPageNow() to drop the
delayed close.

API functions which finish before OnSuspendAck() will not cause this
problem.

Fix:
DecrementLazyKeepAliveCount() should check the is_closing flag before
incrementing close_sequence_id.

BUG=296834
TEST=browser_tests (AppEventPageTest*:LazyBackgroundPageApiTest*)
See bug for how to reproduce and verify issue.

Review URL: https://codereview.chromium.org/24712003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225906 0039d316-1c4b-4281-b951-d872f2087c98
ash: Add OEM wallpaper flags.

Paths to OEM wallpaper images were previously passed via the
--ash-default-wallpaper-* flags. This adds new
--ash-oem-wallpaper-* flags that take precedence over the
default wallpaper flags, so that Chrome can tell whether OEM
wallpaper is present or not.

BUG=298389

Review URL: https://codereview.chromium.org/25046007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225907 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225906
SVN changes up to revision 225907