Fix a test failure caused by the recent change to service_constants.h
Fix a test failure caused by the recent change to service_constants.h

The recent change to service_constants.h
http://gerrit.chromium.org/gerrit/#change,5854 made it
irrelevant to compare string constants by pointers.
We should use StrEq() instead.

TEST=FEATURES=test emerge-x86-alex chromeos-login
BUG=chromium-os:19054

Change-Id: If64d924921285dcd9cbbef3c5884076f4bc9d208
Reviewed-on: http://gerrit.chromium.org/gerrit/5904
Tested-by: Satoru Takabayashi <59864788db41f0f5af51640995670e228eeb7e4d@chromium.org>
Reviewed-by: Ken Mixter <feb6aa3e7f1e44c9811411c1ed2accffd28a2d10@chromium.org>

chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager