Change random number generator for 'tests' to SkMWCRandom. Also removes some

Change random number generator for 'tests' to SkMWCRandom. Also removes some
unused headers and fixes a couple of bugs exposed by changing the random
number generator:

First, the function SkMatrix::getMaxStretch() had an error where it was testing
the square of a number against near-zero. This led to it occasionally taking a
cheaper but imprecise path for computing the eigenvalues of the matrix. It's
been replaced with a check against the square of SK_ScalarNearlyZero.

The second case was a failure in ClipStackTest, where it hit the rare case of
a practically empty clip stack (it has a single Union) and we set a tight
bounds. The bounds rect doesn't get set by GrReducedClip::ReduceClipStack() in
this case, so when it clips the reduced stack it's clipping against garbage,
and the resulting regions don't match. The solution is to initialize the
tightBounds rect.

git-svn-id: http://skia.googlecode.com/svn/trunk@7952 2bbb7eff-a529-9590-31e7-b0007b416f81