GitLab

- Implement a TestDocumentLoader and a TestClient.
- Hook them up to PDFiumEngine to observe its behavior.
- Load a simple 2 page PDF and perform a text search.

Change-Id: I03417880c3b6c3f493f7e08ba07b694272b5b5fc
Reviewed-on: https://chromium-review.googlesource.com/1012976
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553836}