Clip scrollbars to box bounds when they don't fit.

third_party/WebKit/LayoutTests/compositing/overflow/universal-accelerated-overflow-scroll-expected.txt

Index: third_party/WebKit/LayoutTests/compositing/overflow/universal-accelerated-overflow-scroll-expected.txt
diff --git a/third_party/WebKit/LayoutTests/compositing/overflow/universal-accelerated-overflow-scroll-expected.txt b/third_party/WebKit/LayoutTests/compositing/overflow/universal-accelerated-overflow-scroll-expected.txt
index 11370629a0406501e0284b45fd2a792879cb341a..4b5938269692eb1e926e03b66251125d579c77c8 100644
--- a/third_party/WebKit/LayoutTests/compositing/overflow/universal-accelerated-overflow-scroll-expected.txt
+++ b/third_party/WebKit/LayoutTests/compositing/overflow/universal-accelerated-overflow-scroll-expected.txt
@@ -26,6 +26,7 @@
                   ]
                 },
                 {
+                  "bounds": [104, 104],
                   "children": [
                     {
                       "position": [2, 87],
@@ -85,6 +86,7 @@
               ]
             },
             {
+              "bounds": [104, 104],
               "children": [
                 {
                   "position": [2, 87],
@@ -139,6 +141,7 @@
                   ]
                 },
                 {
+                  "bounds": [104, 104],
                   "children": [
                     {
                       "position": [2, 87],
@@ -201,6 +204,7 @@
                   ]
                 },
                 {
+                  "bounds": [104, 104],
                   "children": [
                     {
                       "position": [2, 87],
@@ -260,6 +264,7 @@
               ]
             },
             {
+              "bounds": [104, 104],
               "children": [
                 {
                   "position": [2, 87],
@@ -314,6 +319,7 @@
                   ]
                 },
                 {
+                  "bounds": [104, 104],
                   "children": [
                     {
                       "position": [2, 87],
@@ -372,6 +378,7 @@
               ]
             },
             {
+              "bounds": [104, 104],
               "children": [
                 {
                   "position": [2, 87],
@@ -423,6 +430,7 @@
               ]
             },
             {
+              "bounds": [104, 104],
               "children": [
                 {
                   "position": [2, 87],
@@ -475,6 +483,7 @@
               ]
             },
             {
+              "bounds": [104, 104],
               "children": [
                 {
                   "position": [2, 87],
@@ -533,6 +542,7 @@
               ]
             },
             {
+              "bounds": [104, 104],
               "children": [
                 {
                   "position": [2, 87],
@@ -592,6 +602,7 @@
               ]
             },
             {
+              "bounds": [104, 104],
               "children": [
                 {
                   "position": [2, 87],
@@ -650,6 +661,7 @@
               ]
             },
             {
+              "bounds": [104, 104],
               "children": [
                 {
                   "position": [2, 87],