This change requires the user of LazyInstance to explicitly select
either ::DestructorAtExit or ::Leaky. It seems clear that many users of
LazyInstance have just gone with default and are either unaware or
didn't consider that the default requires running an AtExit at process
shutdown.

As a first step, make this choice explicit, and update current users
to be ::DestructorAtExit to maintain current behaviour. Follow ups can
then attempt to consider individual switches from DestructorAtExit to
Leaky including reviewers with domain-specific knowledge.

i.e. No intended behaviour change from this CL.

R=thakis@chromium.org

Lots of TBRs for mechanical changes:
boliu: android_webview/
danakj: cc/
dmurph: storage/
jam: content/ ipc/, mojo/
jbauman: gpu/
jochen: gin/
oshima: chromeos/
raymes: ppapi/
rdevlin.cronin: extensions/
rockot: device/, services/
rogerta: google_apis/
rsleevi: net/, crypto/
sdefresne: components/, ios/
sergeyu: jingle/
slan: chromecas...