Remove expired histograms Sync.MemoryPressureWarning*

These haven't been used in a while, no reason to keep them.
This also lets us deprecate some prefs that existing only for these
histograms.

Bug: 975739
Change-Id: Icecebf3a1159036c5de6a9d93d5f9f6618f6b489
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715192
Commit-Queue: Marc Treib <[email protected]>
Reviewed-by: Mikel Astiz <[email protected]>
Reviewed-by: Sylvain Defresne <[email protected]>
Reviewed-by: Steven Holte <[email protected]>
Cr-Commit-Position: refs/heads/master@{#680798}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index 17f773f..e4c5ac1d 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -1099,4 +1099,5 @@
   // Added 7/2019.
   syncer::MigrateSyncSuppressedPref(profile_prefs);
   profile_prefs->ClearPref(kSignedInTime);
+  syncer::ClearObsoleteMemoryPressurePrefs(profile_prefs);
 }