Add UMAs about Blink GC metrics

This CL adds the following metrics to UMAs:

- Duration of time consumed in Heap::collectGarbage().
- Duration of time consumed in ThreadState::performPendingSweep().
- The total size of object space in all threads when Heap::collectGarbage() is triggered.

BUG=340522

Review URL: https://codereview.chromium.org/304673003

git-svn-id: svn://svn.chromium.org/blink/trunk@175016 bbb929c8-8fbe-4397-9dbb-9b2b20218538