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SyncPrefs is a thin layer on top of preferences and hosts two groups
of preferences:
1) Actual user-facing settings, such as type-selection, exposed via
   SyncUserSettings.
2) Local "bookkeeping" sync metadata, such the last synced time or the
   client ID (cache GUID).

In addition, there are two cases that fall somewhere in the middle,
whose behavior is changed in this patch:
a) FirstSetupComplete: which roughly represents the user having
   consented to sync-the-feature (as opposed to transport-only upon
   sign-in without explicit user consent).
b) The encryption-bootstrap-token: which represent an explicit
   passphrase (usually custom passphrase) entered by the user, that
   allows decrypting the incoming sync changes and encrypt outgoing
   ones.

The last two preferences above fit group 1 better, so this patch stops
clearing them in SyncPrefs::ClearPreferences(), now renamed to
SyncPrefs::ClearLocalSyncTransportData().

With such cleanup, what used to be
ClearDirectoryConsistencyPreferences() is now merged into a single
clearing function, ClearLocalSyncTransportData(), and all calling sites
are unified by directly clearing prefs in ShutdownImpl(DISABLE_SYNC), as
opposed to individual calling sites.

This introduces -arguably desirable- behavioral changes because
codepaths like RESET_LOCAL_SYNC_DATA or STOP_SYNC_FOR_DISABLED_ACCOUNT
now actually clear all local metadata, which most notably includes
keystore keys.

Change-Id: I2c42f98c4e068c7e340580d0b78a5cd5b5c46171
Bug: 1046237
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2023649
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735841}