[ios] Remove stale image if snapshotting failed

This CL works towards reducing duplicate code in snapshot_generator.mm.
An improvement made in an earlier CL was only made to web view snapshots
and not to native views. This CL fixes native view snapshots.

Bug: 918032
Change-Id: I37d0a9bdead31ccce8e467846c2927105c0a76c7
Reviewed-on: https://chromium-review.googlesource.com/c/1395878Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619972}

ios/chrome/browser/snapshots/snapshot_generator.mm

@@ -116,9 +116,7 @@ BOOL ViewHierarchyContainsWKWebView(UIView* view) {
 
 - (UIImage*)updateSnapshot {
   UIImage* snapshot = [self generateSnapshotWithOverlays:YES];
-  if (snapshot) {
-    [self.snapshotCache setImage:snapshot withSessionID:self.sessionID];
-  }
+  [self updateSnapshotCacheWithImage:snapshot];
   return snapshot;
 }