Add simple cache backend experiment hidden behind a command line option.

The option supports three possible values:
  --use-simple-cache-backend=on
    // Use the simple backend for all caches it supports.
  --use-simple-cache-backend=off (current default)
    // Do not use the simple cache backend.
  --use-simple-cache-backend=experiment (Android-only, will be the new default)
    // Choose dynamically based on the experiment parameters.

We are going to be using it for triggering the simple cache backend
on the current stage of development, later we will convert it to
a true experiment.

BUG=173390
TEST=Make sure simple cache backend runs with:
  chrome --use-simple-cache-backend=on
  # The actual backend triggering will be committed with:
  #   https://codereview.chromium.org/12794003

Review URL: https://chromiumcodereview.appspot.com/12684010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@188634 0039d316-1c4b-4281-b951-d872f2087c98
4 files changed