policy: Remove redundant SetSigninUsername call from  UserPolicySigninServiceBase.

This call was added during a previous refactoring, but seems unnecessary:

PrepareToRegister() is called when the user is performing a signin on a profile, to try to register for policy to see if the user is an enterprise user.

The user has the option at that point to:

a) Create a new profile to complete the signin and load policy
b) Continue signing in with this profile
c) Cancel signin entirely

The call to SetSigninUserName in PrepareToRegister should be unnecessary, since we won't be doing any operations on the CloudPolicyStore except in case b), where we already call InitializeUserCloudPolicyManager() which calls SetSigninUsername().

BUG=271392

Review URL: https://codereview.chromium.org/70233002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234851 0039d316-1c4b-4281-b951-d872f2087c98
Allow PnaclComponentInstaller to overwrite existing install.

This makes it behave more like the DefaultComponentInstaller.
If the existing install does not look right (e.g., wrong
NaCl architecture) we need to be able to overwrite the
existing install with one that is freshly pulled from the
server.

Mac Canary builds of chrome may switch from x86-32 to x86-64.

See bug for other issues (do we want keep some side-by-side copies for a while?).
BUG=316911
R=cpu@chromium.org, waffles@chromium.org

Review URL: https://codereview.chromium.org/69253002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234852 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234852