Treat unknown web app UserDisplayModes as standalone

This CL follows protobuf enum best practices and adds "UNSPECIFIED = 0"
to UserDisplayMode.
https://developers.google.com/protocol-buffers/docs/style#enums

The behavioural change here is that old Chrome clients will interpret
new user display modes (e.g. hypothetically a tabbed window mode) from
the sync server as standalone window mode.
We a more likely to extend the set of window modes rather than browser
tab modes so this is a reasonable fallback.

Change-Id: Ibe989693a6b8fa6b826a0dd8107c5ec71aeb7544
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377671
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802170}

chrome/browser/web_applications/web_app_database.cc

@@ -604,6 +604,10 @@ DisplayMode ToMojomDisplayMode(
   switch (user_display_mode) {
     case ::sync_pb::WebAppSpecifics::BROWSER:
       return DisplayMode::kBrowser;
+    // New display modes will most likely be of the window variety than the
+    // browser tab variety so default to windowed if it's an enum value we don't
+    // know about.
+    case ::sync_pb::WebAppSpecifics::UNSPECIFIED:
     case ::sync_pb::WebAppSpecifics::STANDALONE:
       return DisplayMode::kStandalone;
   }

components/sync/protocol/proto_enum_conversions.cc

@@ -512,9 +512,10 @@ const char* ProtoEnumToString(
 
 const char* ProtoEnumToString(
     sync_pb::WebAppSpecifics::UserDisplayMode user_display_mode) {
-  ASSERT_ENUM_BOUNDS(sync_pb::WebAppSpecifics, UserDisplayMode, BROWSER,
+  ASSERT_ENUM_BOUNDS(sync_pb::WebAppSpecifics, UserDisplayMode, UNSPECIFIED,
                      STANDALONE);
   switch (user_display_mode) {
+    ENUM_CASE(sync_pb::WebAppSpecifics, UNSPECIFIED);
     ENUM_CASE(sync_pb::WebAppSpecifics, BROWSER);
     ENUM_CASE(sync_pb::WebAppSpecifics, STANDALONE);
   }

components/sync/protocol/web_app_specifics.proto

@@ -37,7 +37,7 @@ message WebAppSpecifics {
   // chrome/browser/web_applications/proto/web_app.proto and
   // third_party/blink/public/mojom/manifest/display_mode.mojom
   enum UserDisplayMode {
-    // UNDEFINED is never serialized.
+    UNSPECIFIED = 0;
     BROWSER = 1;
     // MINIMAL_UI is never serialized.
     STANDALONE = 3;