[WK2][Cocoa] Allow overriding the ITP data removal internal using a default

[WK2][Cocoa] Allow overriding the ITP data removal internal using a default
https://bugs.webkit.org/show_bug.cgi?id=174802
<rdar://problem/33497898>

Reviewed by Sam Weinig.

Allow overriding the ITP data removal internal using a default to facilitate testing.
Can be used like so:
defaults write -g ResourceLoadStatisticsMinimumTimeBetweenDataRecordsRemoval 60

Will cause us to write every 60 seconds instead of 3600.

* Shared/WebPreferencesDefinitions.h:
* UIProcess/Cocoa/WebResourceLoadStatisticsStoreCocoa.mm:
(WebKit::WebResourceLoadStatisticsStore::registerUserDefaultsIfNeeded):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@219855 268f45cc-cd09-0410-ab3c-d52691b4dbfc