Move IsDataSaverEnabledByUser to be a static method and use it

This method now officially becomes the source of truth that
everything in the code base eventually calls into to determine whether
or not DataSaver is enabled.

Bug: 934399
Change-Id: Iae837b710ace8cc3101188f79d02cbc2d4f0fd93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1537242
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643948}
PrefService* DataReductionProxySettings::GetOriginalProfilePrefs() {
PrefService* DataReductionProxySettings::GetOriginalProfilePrefs() const {
   DCHECK(thread_checker_.CalledOnValidThread());
   return prefs_;
 }

PrefService* DataReductionProxySettings::GetOriginalProfilePrefs() {
   DCHECK(thread_checker_.CalledOnValidThread());
   return prefs_;
 }
