blink/events: replace an enum constant with an int

We get warnings if floating-point values are multiplied by enums. The
warnings go away if we use a `constexpr int` here instead, which should
be equivalent given `kTickMultiplier`'s current uses.

Bug: 1058218
Change-Id: I5fef7bfc74a9572020e71b62c667d21123135b6c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2096096Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748908}

third_party/blink/renderer/core/events/wheel_event.h

@@ -40,7 +40,7 @@ class CORE_EXPORT WheelEvent final : public MouseEvent {
   DEFINE_WRAPPERTYPEINFO();
 
  public:
-  enum { kTickMultiplier = 120 };
+  constexpr static int kTickMultiplier = 120;
 
   enum DeltaMode { kDomDeltaPixel = 0, kDomDeltaLine, kDomDeltaPage };