Sync: MutableEntry shouldn't track changes that have no actual impact on the stored data

This is a resubmit of https://codereview.chromium.org/1237493002/
that was reverted due to a DEBUG check failure in one of the
tests. I originally added that DCHECK because I believed the
MutableEntry.PutPredecessor call would only be made in a
context of other mutations and that the entry would have to
be already dirty. It turned out there is one particular codepath
where BookmarkChangeProcessor::BookmarkNodeChildrenReordered
gets called that reorders sync bookmarks without doing any
other mutations and might result in failing the DCHECK.
I removed the DCHECK and replaced it with a comment.

Other than that the code change is the same as in the reverted
patch.

Original description:
---------------------

This change optimizes MutableEntry and ModelNeutralMutableEntry
methods that update sync data. Most of these methods compare
new and old values and skip the update when the values match.
However most of these methods still unconditionally register EntryKernel
to be tracked in EntryKernelMutation list by making a TrackChangesTo
call, which immediately makes a deep copy of EntryKernel.
Then there is one more deep copy at the end of the
transaction scope.

This fix avoids the two unnecessary deep copies by calling
TrackChangesTo only when necessary e.g. when the new value
is actually different.

In addition I used this as an opportunity to do a small code cleanup around
marking MutableEntry instances as dirty.

BUG=468144

Review URL: https://codereview.chromium.org/1239393002

Cr-Commit-Position: refs/heads/master@{#339956}

1 files changed, 1 insertions, 1 deletions

diff --git a/sync/syncable/model_neutral_mutable_entry.h b/sync/syncable/model_neutral_mutable_entry.h
index 5ff71ad..76f5c63 100644
--- a/sync/syncable/model_neutral_mutable_entry.h
+++ b/sync/syncable/model_neutral_mutable_entry.h
@@ -100,7 +100,7 @@ class SYNC_EXPORT_PRIVATE ModelNeutralMutableEntry : public Entry {
  protected:
   explicit ModelNeutralMutableEntry(BaseWriteTransaction* trans);
 
-  syncable::MetahandleSet* GetDirtyIndexHelper();
+  void MarkDirty();
 
  private:
   friend class syncer::WriteNode;