Fix flakyness of MetricsService unit tests. 

MetricsService::InitializeMetricsState() has a DCHECK 
that MetricsLog::GetIncrementalUptime() would always 
return 0, which isn't the case for unit tests. (This is 
because MetricsService may be created and destroyed 
multiple times.) 

This CL de-flakes the tests by moving the static 
MetricsLog::GetIncrementalUptime to be a non-static 
MetricsService::GetIncrementalUptime function (with 
the last time stored as an instance variable). 

BUG=290326
TEST=Tests still pass even if you add a sleep() call 
to MetricsServiceTest() constructor.
NOTRY=true

Review URL: https://codereview.chromium.org/25559009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227767 0039d316-1c4b-4281-b951-d872f2087c98
Blink roll 159187:159214

http://build.chromium.org/f/chromium/perf/dashboard/ui/changelog_blink.html?url=/trunk&range=159188:159214&mode=html
TBR=
BUG=

Review URL: https://codereview.chromium.org/26612004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227769 0039d316-1c4b-4281-b951-d872f2087c98
Update .DEPS.git

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227770 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227769
SVN changes up to revision 227770