Install prompt is displayed if widevine is opt opted in by user and user
doesn't choose don't ask option of it.

Widevine cdm lib is loaded into cdm utility process after it installed.
And it is browser widely used not just for specific profile.
However, Brave has prompted widevine permission prompt when user loads
contents site(ex, netflix) in newly created profile.
If Widevine cdm lib is already installed by any profile, Brave should not
display it again because cdm lib is already used.

To fix this, two kWidevineOptedIn/kWidevineInstalledVersion are migrated
from profile prefs to local state.
When migration is needed, firstly active profile is used to get existing
pref values.

There is one more Widevine related pref. It's kAskWidevineInstall.
It would be good to migrate to local prefs also. but left as-is in profile
prefs also seems fine. After Widevine is installed that prefs doesn't have
much meaning and if not just click once to dont ask is sufficient. Also, it
is convinient to implement settings option with profile prefs.

Add WidevinePrefsMigrationTest.PrefMigrationTest for prefs migration
test.

local state is persisted but not persisted after clearing prefs in the
spanning test.
At second test, local state has the value before the clearing.
I expected it has default value because ClearePref() is called
but it's not default value at second test.
So, testing is done by explicitly calling MigrateWidevinePrefs() instead of
spanning test.