Disabled hardware acceleration causing startup problems

[bbondy]

Test cases for QA:

• hardware acceleration should be enabled by default in advanced preferences with a fresh profile
• Turning off hardware acceleration should retain across restarts. Make sure it's actually off by looking at the values in chrome://gpu before and after. It should have more disabled values when off.
• Turn it back on, make sure it works again across application restarts and checking in chrome://gpu
• Use a fresh profile, install 0.19.x and turn off hardware accel. Upgrade to 0.20.x. The first time you start it should be enabled (that's a known bug that we are not going to fix because it's 1 time thing). Every time after that it should be disabled.