[fsp] Make the reading operation abortable.

Reading may be slow. We should be able to abort it to avoid unnecessary
consumption of resources such as network, cpu and memory.

TEST=browser_test: *FileSystemProvider*ReadFile*
BUG=400574

Review URL: https://codereview.chromium.org/496953003

Cr-Commit-Position: refs/heads/master@{#291645}