Explicitly call TestingProfile::Builder::SetIsNewProfile() from tests

Before Chromium 90, both TestingProfile and TestingProfile::Builder did
use base::Optional<bool> for their internal |is_new_profile| attribute,
and calling TestingProfile::IsNewProfile() without having explicitly
provided a value for |is_new_profile| via OverrideIsNewProfile() would
result on returning the value returned by Profile::IsNewProfile():

  bool TestingProfile::IsNewProfile() const {
    if (is_new_profile_.has_value())
      return is_new_profile_.value();
    return Profile::IsNewProfile();
  }

But now that Profile::IsNewProfile() is a pure virtual function, the
|is_new_profile| attribute of TestingProfile is no longer an optional
value, but a bool initialized to false. Therefore, it is now mandatory
to initialize its counterpart in TestingProfile::Builder explicitly in
order to make it clear that we're building a new testing profile.

Chromium change:

https://chromium.googlesource.com/chromium/src.git/+/52208fa33df1dc2174fb044cf777eeecad38838d

commit 52208fa33df1dc2174fb044cf777eeecad38838d
Author: Glen Robertson <glenrob@chromium.org>
Date:   Fri Jan 15 07:10:38 2021 +0000

    Push Profile::IsNewProfile down to subclasses, remove dep on first_run.

    TestingProfile::IsNewProfile now defaults to false instead of defaulting
    to the impl's behaviour when not overridden.

    profile.cc's inclusion of first_run.h was its last dependency on
    //c/b/... except for constants that have separate GN targets, and
    profile_observer. Removing the dependency allows profile.{cc,h} and
    profile_observer.h to be moved from //c/b:browser to a separate GN
    target (will be done in subsequent CL). This will let us break
    dependency cycles and improve modularization of the codebase.

    Bug: 1149277

chromium_src/chrome/browser/signin/account_consistency_disabled_unittest.cc

@@ -31,6 +31,7 @@ TEST(AccountConsistencyDisabledTest, NewProfile) {
   // turned on to 100% of the user base and is no longer needed.
   // See 36417aa39a5e8484b23f1ec927bfda23465f4f21
   TestingProfile::Builder profile_builder;
+  profile_builder.SetIsNewProfile(true);
   {
     TestingPrefStore* user_prefs = new TestingPrefStore();