Adding test helpers so that EXPECT_EQs works and pretty prints output.

Previously;
	../../third_party/WebKit/Source/core/animation/AnimatableLengthTest.cpp:224: Failure
	Value of: testInterpolate(CSSPrimitiveValue::create(createCalc(16, CSSPrimitiveValue::CSS_EMS, -10, CSSPrimitiveValue::CSS_REMS)).get(), from10em.get(), to20rem.get(), -0.5)
	  Actual: false
	Expected: true

Now;
	../../third_party/WebKit/Source/core/animation/AnimatableLengthTest.cpp:226: Failure
	Value of: *(AnimatableValue::interpolate(from10em.get(), to20rem.get(), -0.5).get())
	  Actual: AnimatableLength(calc(15em + -10rem))
	Expected: *(create(16, CSSPrimitiveValue::CSS_EMS, -10, CSSPrimitiveValue::CSS_REMS).get())
	Which is: AnimatableLength(calc(16em + -10rem))

Previously;
	../../third_party/WebKit/Source/core/animation/AnimatableLengthTest.cpp:241: Failure
	Value of: testAdd(CSSPrimitiveValue::create(11, CSSPrimitiveValue::CSS_PX).get(), create(10, CSSPrimitiveValue::CSS_PX).get(), create(0, CSSPrimitiveValue::CSS_MM).get())
	  Actual: false
	Expected: true

Now;
	../../third_party/WebKit/Source/core/animation/AnimatableLengthTest.cpp:244: Failure
	Value of: *(AnimatableValue::add(create(10, CSSPrimitiveValue::CSS_PX).get(), create(0, CSSPrimitiveValue::CSS_MM).get()).get())
	  Actual: AnimatableLength(10px)
	Expected: *(create(11, CSSPrimitiveValue::CSS_PX).get())
	Which is: AnimatableLength(11px)

BUG=

Review URL: https://codereview.chromium.org/26355005

git-svn-id: svn://svn.chromium.org/blink/trunk@159660 bbb929c8-8fbe-4397-9dbb-9b2b20218538