Check and create for Profile README at profile directory creation.

Check and create for Profile README at profile directory creation.

This CL removes disk accesses on profile load by only checking for
the README file when the profile directory is created.

Currently the disk access is done as a separate task 60 seconds after
every profile load, not just during profile directory creation.

Instead, repurpose the check that ensures the profile directory exists, so
that additional work only happens on profile directory creation.

BUG=483362

Review URL: https://codereview.chromium.org/1111313003

Cr-Commit-Position: refs/heads/master@{#334770}