ChromeOS: Remove ProfileHelper::IsSigninProfile.

ChromeOS: Remove ProfileHelper::IsSigninProfile.

Since Profile::IsLoginProfile() was added, ProfileHelper::IsSigninProfile is redundant and can be removed.

With this commit, ProfileImpl::is_login_profile_ is initialized already in the initializer list, so that IsLoginProfile / is_login_profile_ always return the correct value.

BUG=312660

[pneubeck (no reviews), 2013-10-30 21:02:52]

@Mitsuru, FYI

@David, you reviewed the addition of Profile::IsLoginProfile, please also take a
look at this one.

@Nikita, PTAL at ProfileHelper.

[Nikita (slow), 2013-10-30 21:36:02]

lgtm

[pneubeck (no reviews), 2013-10-31 15:12:11]

Ok. After trying to fix all the other profiles like OffTheRecordProfile and
FakeProfile, I come to the conclusion that Profile::IsLoginProfile doesn't have
any advantage compared to ProfileHelper::IsSigninProfile but leads to a lot of
code duplication and GetPath is easily out of sync to IsLoginProfile.

Thus, instead I set up a CL to remove IsLoginProfile again:
https://codereview.chromium.org/47923016