Revert to system Mail app if current default Mail app is no longer there

ios/chrome/browser/web/mailto_url_rewriter_unittest.mm

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/web/mailto_url_rewriter.h"
 
 #import "ios/chrome/browser/web/fake_mailto_handler_helpers.h"
 #import "ios/chrome/browser/web/mailto_handler_system_mail.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/gtest_mac.h"
@@ skipping 37 matching lines @@
   EXPECT_TRUE(rewriter);
   EXPECT_GT([[rewriter defaultHandlerName] length], 0U);
   // ID for system Mail client app must not be an empty string.
   EXPECT_GT([[MailtoURLRewriter systemMailApp] length], 0U);
 
   NSArray<MailtoHandler*>* handlers = [rewriter defaultHandlers];
   EXPECT_GE([handlers count], 1U);
   for (MailtoHandler* handler in handlers) {
     ASSERT_TRUE(handler);
     NSString* appStoreID = [handler appStoreID];
+    NSString* expectedDefaultAppID =
+        [handler isAvailable] ? appStoreID : [MailtoURLRewriter systemMailApp];
     [rewriter setDefaultHandlerID:appStoreID];
-    EXPECT_NSEQ(appStoreID, [rewriter defaultHandlerID]);
+    EXPECT_NSEQ(expectedDefaultAppID, [rewriter defaultHandlerID]);
   }
 }
 
+TEST_F(MailtoURLRewriterTest, TestDefaultsInvalidToSystemMail) {
+  // Sets up a MailtoURLRewriter with 2 MailtoHandler objects:
+  // - system-provided Mail client app
+  // - Gmail app, but it is not installed
+  MailtoURLRewriter* rewriter = [[MailtoURLRewriter alloc] init];
+  MailtoHandler* systemMailHandler = [[MailtoHandlerSystemMail alloc] init];
+  MailtoHandler* fakeGmailHandler =
+      [[FakeMailtoHandlerGmailNotInstalled alloc] init];
+  [rewriter addMailtoApps:@[ systemMailHandler, fakeGmailHandler ]];
+  // Sets the default handler to Gmail (which is not installed). This simulates
+  // the situation when Gmail was installed and set as the default handler.
+  // Then Gmail app is deleted from the device.
+  [rewriter setDefaultHandlerID:[fakeGmailHandler appStoreID]];
+  // Verifies that the system-provided Mail app automatically assumes the
+  // default handler.
+  EXPECT_NSEQ([MailtoURLRewriter systemMailApp], [rewriter defaultHandlerID]);
+}
+
 TEST_F(MailtoURLRewriterTest, TestUserPreferencePersistence) {
   // Sets up a first MailtoURLRewriter with at least 2 MailtoHandler objects.
   // A faked Gmail handler that is installed must be used or -addMailtoApp:
   // will just skip it.
   MailtoURLRewriter* rewriter = [[MailtoURLRewriter alloc] init];
   MailtoHandler* systemMailHandler = [[MailtoHandlerSystemMail alloc] init];
   MailtoHandler* fakeGmailHandler =
       [[FakeMailtoHandlerGmailInstalled alloc] init];
   [rewriter addMailtoApps:@[ systemMailHandler, fakeGmailHandler ]];
 
@@ skipping 147 matching lines @@
       [[FakeMailtoHandlerGmailInstalled alloc] init];
 
   // Sets up a MailtoURLRewriter for testing.
   MailtoURLRewriter* rewriter = [[MailtoURLRewriter alloc] init];
   MailtoHandler* systemMailHandler = [[MailtoHandlerSystemMail alloc] init];
   [rewriter addMailtoApps:@[ systemMailHandler, fakeGmailHandler ]];
 
   // Verify that MailtoURLRewriter will use Gmail app.
   EXPECT_NSEQ(kGmailAppStoreID, [rewriter defaultHandlerID]);
 }