Fix data race in SamplingCircularQueue

This change fixes data race described in the bug by adding Acquire_Load to SamplingCircularQueue::StartDequeue and Acquire_Store to SamplingCircularQueue::Enqueue.

Also the queue implementation imposed a constraint on the records it stored: the first AtomicWord in each record was a marker. For that purpose TickSampleEventRecord had filter field of type int. This approach is error prone, e.g. on x64 sizeof(AtomicWord) is 8 while sizeof(int) is 4. Moreover the queue needs such marker only at the beginning of chunk. I changed the queue so that it stores the marker explicitly as the first Cell in chunk and removed the filter field.

BUG=251218
R=loislo@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/19642002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15750 ce2b1a6d-e550-0410-aec6-3dcde31c8c00