Avoid unnecessary sqrt calls in ComputeTransform2dScaleComponents.

This method performed 2 sqrt calls for every layer during
CalcDrawProperties.  In most cases, only the main diagonal has nonzero
values and the sqrt() call can be optimized out.

I measured a 20% win in CalcDrawPropsImplTest.TenTen on Nexus 4.

NOTRY=true
BUG=311472

Review URL: https://codereview.chromium.org/47403002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231339 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231339