Inline CompareOffsets and disallow null offsets

As pointed out by jbroman@ during a review, null offsets will violate
the requirements of std::sort. Luckily CompareOffsets is only ever used
where offsets will be non-null, so enforce that in the code. Also since
it is used exactly once, just inline it.

Bug: 791086
Change-Id: I9df7f1201fdc3024a727c62bfd7e917791de12b6
Reviewed-on: https://chromium-review.googlesource.com/905100
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534804}

third_party/WebKit/Source/core/animation/Keyframe.cpp

@@ -35,11 +35,4 @@ void Keyframe::AddKeyframePropertiesToV8Object(
   }
 }
 
-bool Keyframe::CompareOffsets(const scoped_refptr<Keyframe>& a,
-                              const scoped_refptr<Keyframe>& b) {
-  if (!a->Offset() || !b->Offset())
-    return false;
-  return a->CheckedOffset() < b->CheckedOffset();
-}
-
 }  // namespace blink

third_party/WebKit/Source/core/animation/Keyframe.h

@@ -186,10 +186,6 @@ class CORE_EXPORT Keyframe : public RefCounted<Keyframe> {
       EffectModel::CompositeOperation effect_composite,
       double offset) const = 0;
 
-  // Comparator function for sorting Keyframes based on their offsets.
-  static bool CompareOffsets(const scoped_refptr<Keyframe>&,
-                             const scoped_refptr<Keyframe>&);
-
  protected:
   Keyframe()
       : offset_(), composite_(), easing_(LinearTimingFunction::Shared()) {}

third_party/WebKit/Source/core/animation/css/CSSAnimations.cpp

@@ -148,8 +148,11 @@ StringKeyframeEffectModel* CreateKeyframeEffectModel(
   }
 
   // Merge duplicate keyframes.
-  std::stable_sort(keyframes.begin(), keyframes.end(),
-                   Keyframe::CompareOffsets);
+  std::stable_sort(
+      keyframes.begin(), keyframes.end(),
+      [](const scoped_refptr<Keyframe>& a, const scoped_refptr<Keyframe>& b) {
+        return a->CheckedOffset() < b->CheckedOffset();
+      });
   size_t target_index = 0;
   for (size_t i = 1; i < keyframes.size(); i++) {
     if (keyframes[i]->CheckedOffset() ==