scheduler: Add a base directory

This patch moves the more fundamental scheduling primitives (e.g.,
TaskQueueManager) to a separate base/ directory. This makes the layered
architecture of the scheduler more obvious and reduces the likelihood of
accidentally introducing layering violations.

With this refactoring, the overall dependencies are:

       +------+
       | base |
       +------+
           ^
           |
       +-------+     +------+
       | child |<----| test |
       +-------+     +------+
          ^ ^          ^ ^
          | |          | |
          | '----------|-|---.
          | .----------' '-. |
          | |              | |
      +----------+      +-------+
      | renderer |      | ppapi |
      +----------+      +-------+

Committed: https://crrev.com/516bee16e9437521cdd20a78d683f9dbaf35df3e
Cr-Commit-Position: refs/heads/master@{#351109}

Review URL: https://codereview.chromium.org/1374653003

Cr-Commit-Position: refs/heads/master@{#351299}
57 files changed