Currently, when a browser process is relaunched, the /prefetch:#
switch of the current process is reused, which is wrong.

For example, when a process initially launched in background is
relaunched in foreground, it shouldn't reuse the /prefetch:#
switch dedicated to background browser processes.

With this CL, RemoveSwitchesForAutostart() removes the
/prefetch:# switch from the command line, if present.

BUG=577698

Review URL: https://codereview.chromium.org/1734053002

Cr-Commit-Position: refs/heads/master@{#378751}