Make an ExtensionApiUnittest class for running API functions

Several different test files are using the same code for running api functions
in a unit test, but they aren't sharing code.  Fix this.

Review URL: https://codereview.chromium.org/131973017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247315 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247315
Replace calls to PO2TS::GetPrimaryAccountId() with SMB::GetAuthenticatedAccountId. 

The functionality is the same, just moving the code to a new place. 

BUG=333995

Review URL: https://codereview.chromium.org/145623003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247316 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247316