Revert "Implement a Hit Test Cache for same point hits."

Source/core/layout/HitTestCache.h

-// Copyright (c) 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef HitTestCache_h
-#define HitTestCache_h
-
-#include "core/layout/HitTestResult.h"
-#include "wtf/Vector.h"
-
-namespace blink {
-
-// This object implements a cache for storing successful hit tests to DOM nodes
-// in the visible viewport. The cache is cleared on dom modifications,
-// scrolling, CSS style modifications.
-//
-// Multiple hit tests can occur when processing events. Typically the DOM doesn't
-// change when each event is processed so in order to decrease the time spent
-// processing the events a hit cache is useful. For example a GestureTap event
-// will generate a series of simulated mouse events (move, down, up, click)
-// with the same co-ordinates and ideally we'd like to do the hit test once
-// and use the result for the targetting of each event.
-//
-// Some of the related design, motivation can be found in:
-// https://docs.google.com/document/d/1b0NYAD4S9BJIpHGa4JD2HLmW28f2rUh1jlqrgpU3zVU/
-//
-
-// A cache size of 2 is used because it is relatively cheap to store;
-// and the ping-pong behaviour of some of the HitTestRequest flags during
-// Mouse/Touch/Pointer events can generate increased cache misses with
-// size of 1.
-#define HIT_TEST_CACHE_SIZE (2)
-
-class HitTestCache {
-public:
-    HitTestCache() = default;
-
-    // Check the cache for a possible hit and update |result| if
-    // hit encountered; returning true. Otherwise false.
-    bool lookupCachedResult(HitTestResult&, uint64_t domTreeVersion);
-
-    void clear();
-
-    // Verify that the |actual| object matches the |expected| object; and
-    // log UMA metrics indicating the result.
-    static void verifyCachedResult(const HitTestResult& expected, const HitTestResult& actual);
-
-    // Adds a HitTestResult to the cache.
-    void addCachedResult(const HitTestResult&, uint64_t domTreeVersion);
-
-private:
-    // These values are reported in UMA as the "EventHitTest" enumeration.
-    // Do not reorder, append new values at the end, deprecate old
-    // values and update histograms.xml.
-    enum class HitHistogramMetric {
-        MISS, // Miss, not found in cache.
-        MISS_EXPLICIT_AVOID, // Miss, calle asked to explicitly avoid cache.
-        MISS_VALIDITY_RECT_MATCHES, // Miss, validity region matches, type doesn't.
-        HIT_EXACT_MATCH, // Hit, exact point matches.
-        HIT_REGION_MATCH, // Hit, validity region matches.
-        MAX_HIT_METRIC = HIT_REGION_MATCH,
-    };
-
-    // These values are reported in UMA as the "EventHitTestValidity"
-    // enumeration. Do not reorder, append new values at the end,
-    // deprecate old values and update histograms.xml.
-    enum class ValidityHistogramMetric {
-        VALID_EXACT_MATCH, // Correct node for exact point test.
-        VALID_REGION, // Correct node for region check.
-        INCORRECT_RECT_BASED_EXACT_MATCH, // Wrong node returned for cache hit with point was exact match and rect based test.
-        INCORRECT_POINT_EXACT_MATCH, // Wrong node returned for cache hit with exact point match and was explicit point test.
-        INCORRECT_RECT_BASED_REGION, // Wrong node returned for rect with region matching and was rect based test.
-        INCORRECT_POINT_REGION, // Wrong node returned for point with region matching and was explicit point test.
-        MAX_VALIDITY_METRIC = INCORRECT_POINT_REGION,
-    };
-
-    unsigned m_updateIndex = 0;
-    WillBeHeapVector<HitTestResult, HIT_TEST_CACHE_SIZE> m_items;
-    uint64_t m_domTreeVersion = 0;
-};
-
-} // namespace blink
-
-#endif // HitTestCache_h