Linux: Correctly count the number of open file descriptors.

The initial implementation of ProcessMetrics::GetOpenFdCount() counted
"." and ".." as well.

Add a unit test to verify the counting is correct.

BUG=496957

Review URL: https://codereview.chromium.org/1526253003

Cr-Commit-Position: refs/heads/master@{#365504}
4 files changed