Cleanup: Rename NumberOfAlgorithmOperations --> LastAlgorithmOperation.

This avoids having to handle the sentinel enum value inside of switch statements.

BUG=245025

Review URL: https://codereview.chromium.org/105433002

git-svn-id: svn://svn.chromium.org/blink/trunk@163304 bbb929c8-8fbe-4397-9dbb-9b2b20218538