Revert "Replaced css inject hack in TestUtils with Page.Styles.add() #11798"

Removing this change to make the tests behave more nicely until #11848 has been fixed.

This reverts commit f2c2236cf8d0428f100e17586f7946095ffea158

Change-Id: I776699f5082273e9c4d11248cc35e7ed6b665873