[compiler] Readability refactor of comilation pipeline.

This is a pure refactoring CL and should not contain any functional
changes to the code. The following has been done:
- Group compiler.cc into sections for each component.
- Surround local helper methods by anonymous namespace.
- Move implementation of Compiler (API class) together.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1809403002

Cr-Commit-Position: refs/heads/master@{#34891}