[ios] Don't convert an empty gfx::Image to a UIImage.

WKWebView -takeSnapshotWithConfiguration can return a nil
UIImage if there is an error.

Change-Id: Ifb1de97f9afed7436b5fae655ac9a35facdc2868
Reviewed-on: https://chromium-review.googlesource.com/c/1316750Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606047}

ios/chrome/browser/snapshots/snapshot_generator.mm

@@ -262,11 +262,14 @@ BOOL ViewHierarchyContainsWKWebView(UIView* view) {
         SnapshotGenerator* strongSelf = weakSelf;
         if (!strongSelf || !_webState)
           return;
-        UIImage* snapshot = image.ToUIImage();
-        if (overlays.count > 0) {
-          snapshot = [strongSelf snapshotWithOverlays:overlays
-                                             snapshot:snapshot
-                                                frame:snapshotFrame];
+        UIImage* snapshot = nil;
+        if (!image.IsEmpty()) {
+          snapshot = image.ToUIImage();
+          if (overlays.count > 0) {
+            snapshot = [strongSelf snapshotWithOverlays:overlays
+                                               snapshot:snapshot
+                                                  frame:snapshotFrame];
+          }
         }
         [strongSelf.snapshotCache setImage:snapshot
                              withSessionID:_snapshotSessionId];