Revert "[iOS] Disabling flaky sync tests"

This reverts commit fb745f66.

Reason for revert: underlying issue no longer reproduces on
other platforms after recent improvements, so optimistically
assuming it was also fixed for iOS.

Original change's description:
> [iOS] Disabling flaky sync tests
> 
> Disabling EarlGrey tests from SigninInteractionControllerTestCase:
>  + testSignInSwitchAccountsAndKeepDataSeparate
>  + testSignInSwitchAccountsAndImportData
>  + testSignInSwitchManagedAccount
> 
> Those tests seem to trigger an issue in the sync code when doing enable
> disable enable too fast.
> 
> Bug: 854446
> Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
> Change-Id: I76d10d272297e91650dbe91ef7acee500eac6c75
> Reviewed-on: https://chromium-review.googlesource.com/1107881
> Reviewed-by: Justin Cohen <justincohen@chromium.org>
> Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#568843}

TBR=justincohen@chromium.org,jlebel@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 854446
Change-Id: Ic809289ce4662d099250668d6f23100e52de9587
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Reviewed-on: https://chromium-review.googlesource.com/1114719Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570374}

ios/chrome/browser/ui/signin_interaction/signin_interaction_controller_egtest.mm

@@ -99,8 +99,7 @@ void WaitForMatcher(id<GREYMatcher> matcher) {
 
 // Tests signing in with one account, switching sync account to a second and
 // choosing to keep the browsing data separate during the switch.
-// TODO(crbug.com/854446): Enable after fixing.
-- (void)DISABLED_testSignInSwitchAccountsAndKeepDataSeparate {
+- (void)testSignInSwitchAccountsAndKeepDataSeparate {
   // Set up the fake identities.
   ios::FakeChromeIdentityService* identity_service =
       ios::FakeChromeIdentityService::GetInstanceFromChromeProvider();
@@ -137,8 +136,7 @@ void WaitForMatcher(id<GREYMatcher> matcher) {
 
 // Tests signing in with one account, switching sync account to a second and
 // choosing to import the browsing data during the switch.
-// TODO(crbug.com/854446): Enable after fixing.
-- (void)DISABLED_testSignInSwitchAccountsAndImportData {
+- (void)testSignInSwitchAccountsAndImportData {
   // Set up the fake identities.
   ios::FakeChromeIdentityService* identity_service =
       ios::FakeChromeIdentityService::GetInstanceFromChromeProvider();
@@ -176,8 +174,7 @@ void WaitForMatcher(id<GREYMatcher> matcher) {
 
 // Tests that switching from a managed account to a non-managed account works
 // correctly and displays the expected warnings.
-// TODO(crbug.com/854446): Enable after fixing.
-- (void)DISABLED_testSignInSwitchManagedAccount {
+- (void)testSignInSwitchManagedAccount {
   // Set up the fake identities.
   ios::FakeChromeIdentityService* identity_service =
       ios::FakeChromeIdentityService::GetInstanceFromChromeProvider();