[iOS] Accept nil callback in ChromeIdentityService::GetAvatarForIdentity()

Adding comments for ChromeIdentityService to accept nil as |callback| for
ChromeIdentityService::GetAvatarForIdentity().
And Adding support for nil callback in FakeChromeIdentityService.

Related to: crrev.com/i/1296954

Change-Id: I0d96d8a3abcd934705ec7743d7711b760ea1b459
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609842
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659480}

ios/public/provider/chrome/browser/signin/chrome_identity_service.h

@@ -167,7 +167,7 @@ class ChromeIdentityService {
 
   // Fetches the profile avatar, from the cache or the network.
   // For high resolution iPads, returns large images (200 x 200) to avoid
-  // pixelization. Calls back on the main thread.
+  // pixelization. Calls back on the main thread. |callback| may be nil.
   virtual void GetAvatarForIdentity(ChromeIdentity* identity,
                                     GetAvatarCallback callback);
 

ios/public/provider/chrome/browser/signin/fake_chrome_identity_service.mm

@@ -29,15 +29,6 @@ UIImage* FakeGetCachedAvatarForIdentity(ChromeIdentity*) {
   return provider ? provider->GetDefaultAvatar() : nil;
 }
 
-void FakeGetAvatarForIdentity(ChromeIdentity* identity,
-                              ios::GetAvatarCallback callback) {
-  // |GetAvatarForIdentity| is normally an asynchronous operation, this is
-  // replicated here by dispatching it.
-  dispatch_async(dispatch_get_main_queue(), ^{
-    callback(FakeGetCachedAvatarForIdentity(identity));
-  });
-}
-
 void FakeGetHostedDomainForIdentity(ChromeIdentity* identity,
                                     ios::GetHostedDomainCallback callback) {
   NSString* domain = base::SysUTF8ToNSString(gaia::ExtractDomainName(
@@ -230,7 +221,14 @@ UIImage* FakeChromeIdentityService::GetCachedAvatarForIdentity(
 void FakeChromeIdentityService::GetAvatarForIdentity(
     ChromeIdentity* identity,
     GetAvatarCallback callback) {
-  FakeGetAvatarForIdentity(identity, callback);
+  if (!callback) {
+    return;
+  }
+  // |GetAvatarForIdentity| is normally an asynchronous operation, this is
+  // replicated here by dispatching it.
+  dispatch_async(dispatch_get_main_queue(), ^{
+    callback(FakeGetCachedAvatarForIdentity(identity));
+  });
 }
 
 void FakeChromeIdentityService::GetHostedDomainForIdentity(