Revert "Revert 154194 "Manage FloatingObjects using hashmaps""

This reverts commit r155275. It caused hundreds of layout test failures,
e.g. http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%2032/builds/12076.

TBR=inferno@chromium.org

Review URL: https://codereview.chromium.org/21438002

git-svn-id: svn://svn.chromium.org/blink/trunk@155282 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add a UseCounter for -webkit-line-clamp.

The CSS UseCounter shows that this property is used on 0.5% of pages,
which seems unbelievably high. Since this feature only does something
in deprecated flexbox, lets measure instead the actual application
of the property to the render tree.

Review URL: https://chromiumcodereview.appspot.com/21298005

git-svn-id: svn://svn.chromium.org/blink/trunk@155283 bbb929c8-8fbe-4397-9dbb-9b2b20218538