Fix comment for kPasswordStore in chrome_switches.cc

The comment specified the purpose and possible values of the --password-store
switch. It was outdated in both respects. Since the proper implementation of
os_crypt on Linux earlier this year, the storage was used not just for
passwords, but also for os_crypt's encryption key.

This is even more problematic because the comment is picked up to generate
http://peter.sh/experiments/chromium-command-line-switches/.

This CL updates the description based on the relevant code in
components/os_crypt/key_storage_util_linux.cc.

The CL does not change the name of the flag on purpose: it is likely contained
in a lot of testing and developers' set-ups, and changing it could have
unwanted side-effects of testing data suddenly piling up in people's
Keyrings/KWallets. Once the alternative backends for Linux will stop being
used for storing passwords (https://crbug.com/571003), and only for the single
encryption key of os_crypt, it will be much safer to rename the switch.

BUG=673121

Review-Url: https://codereview.chromium.org/2563253002
Cr-Commit-Position: refs/heads/master@{#437880}

chrome/common/chrome_switches.cc

@@ -1018,8 +1018,14 @@ const char kOpenAsh[]                       = "open-ash";
 const char kHelp[]                          = "help";
 const char kHelpShort[]                     = "h";
 
-// Specifies which password store to use
-// (detect, default, gnome-keyring, gnome-libsecret, kwallet).
+// Specifies which encryption storage backend to use. Possible values are
+// kwallet, kwallet5, gnome, gnome-keyring, gnome-libsecret, basic. Any other
+// value will lead to Chrome detecting the best backend automatically.
+// TODO(crbug.com/571003): Once PasswordStore no longer uses the Keyring or
+// KWallet for storing passwords, rename this flag to stop referencing
+// passwords. Do not rename it sooner, though; developers and testers might
+// rely on it keeping large amounts of testing passwords out of their Keyrings
+// or KWallets.
 const char kPasswordStore[]                 = "password-store";
 
 // The same as the --class argument in X applications.  Overrides the WM_CLASS