Revert "Reland "[iOS] Disabling flaky sync tests""

This reverts commit e6aa52bf.

Reason for revert: recent patches fix a related problem, in
23919ce6. We haven't been able
to confirm the fix being effective for the flakes observed in
bots because they no longer repro locally, but it is definitely
related so let's reenable the tests and monitor flakes.

Original change's description:
> 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/1117578
> Reviewed-by: Mikel Astiz <mastiz@chromium.org>
> Commit-Queue: Mikel Astiz <mastiz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#570888}

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: I5874890bed60c173c35e49fe86933a06bd214120
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Reviewed-on: https://chromium-review.googlesource.com/1126260Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572570}

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();