Clean up JSConstructStub

Clean up JSConstructStub

- fix truthfulness of comments
- use InitializeFieldsWithFiller more consistently
- use unsigned comparisons for pointers

No change in functionality intended.

Bonus: improve JavaScriptFrame::Print() for an enhanced debugging experience:

- print PC of each frame
- print the function's source also for optimized frames

Review URL: https://codereview.chromium.org/1186823003

Cr-Commit-Position: refs/heads/master@{#29082}