[iOS] Fix startup crash from thumb strip snapshotting

This crash only appears when certain other flags are disabled and the
thumb strip is enabled (I didn't go through to find out exactly which
ones). The root cause is that with these flags disabled, the thumb strip
is created before the BVC's view is loaded, so the tab strip view has
not been created yet either.

Bug: 1094335
Change-Id: If70a670f366145514998ca5e1713a18c7770f58a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2608832Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#840169}

ios/chrome/browser/ui/browser_view/browser_view_controller.mm

@@ -2879,18 +2879,24 @@ NSString* const kBrowserViewControllerSnackbarCategory =
   // parent is hidden but the view itself is not, the snapshot will not be a
   // blank view.
   [self.tabStripSnapshot removeFromSuperview];
-  self.tabStripSnapshot = [self.tabStripView screenshotForAnimation];
-  self.tabStripSnapshot.translatesAutoresizingMaskIntoConstraints = NO;
-  self.tabStripSnapshot.transform =
-      currentViewRevealState == ViewRevealState::Hidden
-          ? [self.tabStripView adjustTransformForRTL:CGAffineTransformIdentity]
-          : [self.tabStripView
-                adjustTransformForRTL:CGAffineTransformMakeTranslation(
-                                          0,
-                                          self.tabStripView.frame.size.height)];
-  self.tabStripView.hidden = YES;
-  [self.contentArea addSubview:self.tabStripSnapshot];
-  AddSameConstraints(self.tabStripSnapshot, self.tabStripView);
+  // During initial setup, the tab strip view may be nil, but the missing
+  // snapshot will never be visible because all three animation methods are
+  // called in succession.
+  if (self.tabStripView) {
+    self.tabStripSnapshot = [self.tabStripView screenshotForAnimation];
+    self.tabStripSnapshot.translatesAutoresizingMaskIntoConstraints = NO;
+    self.tabStripSnapshot.transform =
+        currentViewRevealState == ViewRevealState::Hidden
+            ? [self.tabStripView
+                  adjustTransformForRTL:CGAffineTransformIdentity]
+            : [self.tabStripView
+                  adjustTransformForRTL:CGAffineTransformMakeTranslation(
+                                            0, self.tabStripView.frame.size
+                                                   .height)];
+    self.tabStripView.hidden = YES;
+    [self.contentArea addSubview:self.tabStripSnapshot];
+    AddSameConstraints(self.tabStripSnapshot, self.tabStripView);
+  }
 
   // Remove the fake status bar to allow the thumb strip animations to appear.
   [_fakeStatusBarView removeFromSuperview];