Reapply "Let Layout::GetSupportedScaleFactorFromScale() return only supported ScaleFactor."

Reapply "Let Layout::GetSupportedScaleFactorFromScale() return only supported ScaleFactor."

Original Review URL: https://chromiumcodereview.appspot.com/10946019
Reverted in: https://codereview.chromium.org/10959035

Removed LayoutTest.GetScaleFactorFromScaleNoneSupported() in the original patch because
GetSupportedScaleFactors() will initialize the supported scale factors
to default after test::SetSupportedScaleFactors(std::vector<ScaleFactor>()).
And it's also not a real scenario.

BUG=148841
TBR=sky,pkotwicz

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=157935

[sky, 2012-09-21 16:14:00]

Can you upload the patch you landed, then the fix so we can easily see what
changed?

[Xianzhu, 2012-09-24 17:44:48]

On 2012/09/21 16:14:00, sky wrote:
> Can you upload the patch you landed, then the fix so we can easily see what
> changed?

Sorry for landing the fixed patch without uploading the original patch and a
good description. Just uploaded the patches and updated the description here.
I'll remember to do them the next time.

[sky, 2012-09-24 20:55:55]

LGTM