Replace "deprecated" with "legacy" in IMPLICIT_PASSPHRASE comment

Although new users can't enter this state, a number of users still
remain on it. "legacy" probably reflects better this scenario.

Bug: None
Change-Id: I1e61f6ed601bc6ea8bb51490ab5ebc00d4dd7efc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2312803Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/master@{#791183}

components/sync/protocol/nigori_specifics.proto

@@ -105,7 +105,7 @@ message NigoriSpecifics {
     // Proto enums should begin with an 'unknown' value for switch statements,
     // unknown fields, etc.
     UNKNOWN = 0;
-    // Gaia-based encryption passphrase. Deprecated.
+    // Gaia-based encryption passphrase. Legacy.
     IMPLICIT_PASSPHRASE = 1;
     // Keystore key encryption passphrase. Uses |keystore_bootstrap| to
     // decrypt |encryption_keybag|.