Reland "[iOS] Disabling flaky sync tests"

This reverts commit 661767b9.

Reason for revert: tests still flaky.

Original change's description:
> 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/1114719
> Reviewed-by: Mikel Astiz <mastiz@chromium.org>
> Commit-Queue: Mikel Astiz <mastiz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#570374}

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

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

Bug: 854446
Change-Id: I2a0a2e9d60988783fca93ef8313594d57b7ab906
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Reviewed-on: https://chromium-review.googlesource.com/1117578Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570888}

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

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