Assert GetInterpolableValue is a length before attempting to cast it.

Instead of just expecting, which doesn't stop executing if it isn't.

TBR=xiaochengh@chromium.org

Bug: 1117509
Change-Id: I058f8fd69d8dbebad60b9625c6dfda112eb0b5db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363227Reviewed-by: Chris Sharp <csharp@chromium.org>
Commit-Queue: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799217}

third_party/blink/renderer/core/animation/keyframe_effect_model_test.cc

@@ -74,7 +74,7 @@ class AnimationKeyframeEffectModel : public PageTestBase {
             ->GetCachedValueForTesting();
     // Length values are stored as an |InterpolableLength|; here we assume
     // pixels.
-    EXPECT_TRUE(typed_value->GetInterpolableValue().IsLength());
+    ASSERT_TRUE(typed_value->GetInterpolableValue().IsLength());
     const InterpolableLength& length =
         To<InterpolableLength>(typed_value->GetInterpolableValue());
     // Lengths are computed in logical units, which are quantized to 64ths of