TestPlugin to chromium c++ style.

Changes:
1) Run clang-format through source and header files.
2) Rename data member variables to use unix_hacker_ style.
3) Rename methods to use CamelCase style.
4) Rename file name to test_plugin.

BUG=331299

Review URL: https://codereview.chromium.org/469943003

Cr-Commit-Position: refs/heads/master@{#293160}
7 files changed