Extensions: Remove the "signature" manifest key.

This is an unused manifest key which hasn't been used for
years. It seems this was used for signature verification:
https://codereview.chromium.org/118328/patch/1205/130 but at
some point it's use was removed.

TODO: Also remove this key from
https://developer.chrome.com/docs/extensions/mv2/manifest/ and
https://developer.chrome.com/docs/extensions/mv3/manifest/.

BUG=None

Change-Id: I4ed42510e169c2fca569362820ebb210a1082845
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622572Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843235}

chrome/browser/chromeos/extensions/device_local_account_management_policy_provider.cc

@@ -218,8 +218,8 @@ const char* const kSafeManifestEntries[] = {
     // Just a display string.
     emk::kShortName,
 
-    // Doc missing. Declared as a feature, but unused.
-    // emk::kSignature,
+    // Deprecated manifest key.
+    // "signature",
 
     // Network access.
     emk::kSockets,

chrome/common/extensions/api/_manifest_features.json

@@ -209,10 +209,6 @@
     "channel": "stable",
     "extension_types": "all"
   },
-  "signature": {
-    "channel": "stable",
-    "extension_types": "all"
-  },
   "storage": {
     "channel": "stable",
     "extension_types": [

extensions/common/manifest_constants.cc

@@ -134,7 +134,6 @@ const char kSettingsOverrideAlternateUrls[] =
 const char kSharedModuleAllowlist[] = "allowlist";
 const char kSharedModuleLegacyAllowlist[] = "whitelist";
 const char kShortName[] = "short_name";
-const char kSignature[] = "signature";
 const char kSockets[] = "sockets";
 const char kStorageManagedSchema[] = "storage.managed_schema";
 const char kSuggestedKey[] = "suggested_key";
@@ -586,8 +585,6 @@ const char kInvalidSearchEngineURL[] =
     "Invalid URL [*] for 'chrome_settings_overrides.search_provider'.";
 const char kInvalidShortName[] =
     "Invalid value for 'short_name'.";
-const char kInvalidSignature[] =
-    "Value 'signature' is missing or invalid.";
 const char kInvalidStartupOverrideURL[] =
     "Invalid value for overriding startup URL: '[*]'.";
 const char kInvalidSystemIndicator[] =

extensions/common/manifest_constants.h

@@ -136,7 +136,6 @@ extern const char kSettingsOverrideAlternateUrls[];
 extern const char kSharedModuleAllowlist[];
 extern const char kSharedModuleLegacyAllowlist[];
 extern const char kShortName[];
-extern const char kSignature[];
 extern const char kSockets[];
 extern const char kStorageManagedSchema[];
 extern const char kSuggestedKey[];
@@ -416,7 +415,6 @@ extern const char kInvalidSandboxedPage[];
 extern const char kInvalidSearchEngineMissingKeys[];
 extern const char kInvalidSearchEngineURL[];
 extern const char kInvalidShortName[];
-extern const char kInvalidSignature[];
 extern const char kInvalidStartupOverrideURL[];
 extern const char kInvalidSystemIndicator[];
 extern const char kInvalidTheme[];