Use correct proportion when computing the thumb length

This CL checks used total size before computing the thumb
size. If it's zero, we'll show no thumb.

BUG=421869

Review URL: https://codereview.chromium.org/662593004

git-svn-id: svn://svn.chromium.org/blink/trunk@183919 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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

@@ -268,7 +268,11 @@ int ScrollbarTheme::thumbLength(ScrollbarThemeClient* scrollbar)
         overhang = -scrollbar->currentPos();
     else if (scrollbar->visibleSize() + scrollbar->currentPos() > scrollbar->totalSize())
         overhang = scrollbar->currentPos() + scrollbar->visibleSize() - scrollbar->totalSize();
-    float proportion = (scrollbar->visibleSize() - overhang) / usedTotalSize(scrollbar);
+    float proportion = 0.0f;
+    float totalSize = usedTotalSize(scrollbar);
+    if (totalSize > 0.0f) {
+        proportion = (scrollbar->visibleSize() - overhang) / totalSize;
+    }
     int trackLen = trackLength(scrollbar);
     int length = round(proportion * trackLen);
     length = std::max(length, minimumThumbLength(scrollbar));