Revert "Fix overlay scrollbar at huge scroll offsets"

This reverts commit 9e21c73c.

Reason for revert: suspect causing WebKit Android (Nexus 4) to fail on webkit_unit_tests
https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Android%20%28Nexus4%29/builds/67957

Original change's description:
> Fix overlay scrollbar at huge scroll offsets
> 
> The overlay scrollbar ThumbPosition method was losing precision at very
> high scroll offsets. This would cause it to drift away from where we
> actually show the thumb from the compositor so the hit testing would be
> unreliable.
> 
> Looking at this method, there's no reason it should need to be any
> different from the base class ScrollbarTheme::ThumbPosition which is
> robust under high scroll offsets so the solution here is to simply
> remove the override. Less code, less bugs.
> 
> Bug: 739668
> Change-Id: Ib90152d989931cd0bcc0bddfb226d97d577ce575
> Reviewed-on: https://chromium-review.googlesource.com/598734
> Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
> Reviewed-by: Jeremy Roman <jbroman@chromium.org>
> Commit-Queue: David Bokan <bokan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#491766}

TBR=bokan@chromium.org,jbroman@chromium.org,dtapuska@chromium.org

Change-Id: I44f227bc5956899ff43e7c6d96e7c5596666ae36
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 739668
Reviewed-on: https://chromium-review.googlesource.com/600848Reviewed-by: Luna Lu <loonybear@chromium.org>
Commit-Queue: Luna Lu <loonybear@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491798}

third_party/WebKit/Source/core/layout/ScrollbarsTest.cpp

@@ -203,38 +203,6 @@ TEST_P(ScrollbarAppearanceTest, ThemeEngineDefinesMinimumThumbLength) {
   EXPECT_EQ(StubWebThemeEngine::kMinimumVerticalLength,
             theme.ThumbLength(*scrollable_area->VerticalScrollbar()));
 }
-
-// Ensure thumb position is correctly calculated even at ridiculously large
-// scales.
-TEST_P(ScrollbarAppearanceTest, HugeScrollingThumbPosition) {
-  ScopedTestingPlatformSupport<ScrollbarTestingPlatformSupport> platform;
-
-  v8::HandleScope handle_scope(v8::Isolate::GetCurrent());
-  WebView().Resize(WebSize(1000, 1000));
-  SimRequest request("https://example.com/test.html", "text/html");
-  LoadURL("https://example.com/test.html");
-  request.Complete(
-      "<style> body { margin: 0px; height: 10000000px; } </style>");
-  ScrollableArea* scrollable_area =
-      GetDocument().View()->LayoutViewportScrollableArea();
-
-  Compositor().BeginFrame();
-
-  scrollable_area->SetScrollOffset(ScrollOffset(0, 10000000),
-                                   kProgrammaticScroll);
-
-  int scroll_y = scrollable_area->GetScrollOffset().Height();
-  ASSERT_EQ(9999000, scroll_y);
-
-  Scrollbar* scrollbar = scrollable_area->VerticalScrollbar();
-  ASSERT_TRUE(scrollbar);
-
-  int maximumThumbPosition =
-      WebView().Size().height - StubWebThemeEngine::kMinimumVerticalLength;
-
-  EXPECT_EQ(maximumThumbPosition,
-            scrollbar->GetTheme().ThumbPosition(*scrollbar));
-}
 #endif
 
 }  // namespace

third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.cpp

@@ -103,6 +103,17 @@ double ScrollbarThemeOverlay::OverlayScrollbarFadeOutDurationSeconds() const {
   return style.fade_out_duration_seconds;
 }
 
+int ScrollbarThemeOverlay::ThumbPosition(const ScrollbarThemeClient& scrollbar,
+                                         float scroll_position) {
+  if (!scrollbar.TotalSize())
+    return 0;
+
+  int track_len = TrackLength(scrollbar);
+  float proportion =
+      static_cast<float>(scroll_position) / scrollbar.TotalSize();
+  return round(proportion * track_len);
+}
+
 int ScrollbarThemeOverlay::ThumbLength(const ScrollbarThemeClient& scrollbar) {
   int track_len = TrackLength(scrollbar);
 

third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.h

@@ -59,6 +59,8 @@ class PLATFORM_EXPORT ScrollbarThemeOverlay : public ScrollbarTheme {
   double OverlayScrollbarFadeOutDelaySeconds() const override;
   double OverlayScrollbarFadeOutDurationSeconds() const override;
 
+  int ThumbPosition(const ScrollbarThemeClient&,
+                    float scroll_position) override;
   int ThumbLength(const ScrollbarThemeClient&) override;
 
   bool HasButtons(const ScrollbarThemeClient&) override { return false; }