[ios] [BVC refactor] Prevent crash in BrowserCoordinator |-stop|

A coordinator's |-stop| should not crash if called multiple times.

Bug: 910652
Change-Id: I5a2998726ad3390a8b1c98afbc753f093a4a8986
Reviewed-on: https://chromium-review.googlesource.com/c/1357001
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612706}

ios/chrome/browser/ui/main/browser_coordinator.mm

@@ -47,6 +47,9 @@
                                   RepostFormTabHelperDelegate,
                                   WebStateListObserving>
 
+// Whether the coordinator is started.
+@property(nonatomic, assign, getter=isStarted) BOOL started;
+
 // Handles command dispatching.
 @property(nonatomic, strong) CommandDispatcher* dispatcher;
 
@@ -105,6 +108,9 @@
 #pragma mark - ChromeCoordinator
 
 - (void)start {
+  if (self.started)
+    return;
+
   DCHECK(self.browserState);
   DCHECK(!self.viewController);
   self.dispatcher = [[CommandDispatcher alloc] init];
@@ -116,9 +122,12 @@
   [self installDelegatesForAllWebStates];
   [self addWebStateListObserver];
   [super start];
+  self.started = YES;
 }
 
 - (void)stop {
+  if (!self.started)
+    return;
   [super stop];
   [self removeWebStateListObserver];
   [self uninstallDelegatesForAllWebStates];
@@ -126,6 +135,7 @@
   [self stopChildCoordinators];
   [self destroyViewController];
   self.dispatcher = nil;
+  self.started = NO;
 }
 
 #pragma mark - Public