Refactor GetBrowserModelId so it doesn't call BuildAppModelId.

GetChromiumModelIdForProfile passes the result of GetBrowserModelId to
BuildAppModelId, but GetBrowserModelId calls BuildAppModelId.
Conceptually, it's cleaner if BuildAppModelId is always the last step in
computing an app model id. To do this, we have to extract out a method
to shorten an app model id component, so that GetBrowserModelId can use
it, instead of using BuildAppModelId.

Change-Id: I633c41b83bfc14c52bb4b5e364bce8dfc68cdde2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2304162
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789976}

chrome/installer/util/shell_util.cc

@@ -1482,6 +1482,15 @@ bool RemoveShortcutFolderIfEmpty(ShellUtil::ShortcutLocation location,
   return true;
 }
 
+// Return a shortened version of |component|. Cut in the middle to try
+// to avoid losing the unique parts of |component| (which are usually
+// at the beginning or end for things like usernames and paths).
+base::string16 ShortenAppModelIdComponent(const base::string16& component,
+                                          int desired_length) {
+  return component.substr(0, desired_length / 2) +
+         component.substr(component.length() - ((desired_length + 1) / 2));
+}
+
 }  // namespace
 
 const wchar_t* ShellUtil::kRegDefaultIcon = L"\\DefaultIcon";
@@ -1856,11 +1865,10 @@ base::string16 ShellUtil::GetBrowserModelId(bool is_per_user_install) {
   } else if (is_per_user_install && !GetUserSpecificRegistrySuffix(&suffix)) {
     NOTREACHED();
   }
-  // There is only one component (i.e. the suffixed appid) in this case, but it
-  // is still necessary to go through the appid constructor to make sure the
-  // returned appid is truncated if necessary.
-  std::vector<base::string16> components(1, app_id.append(suffix));
-  return BuildAppModelId(components);
+  app_id.append(suffix);
+  if (app_id.length() <= installer::kMaxAppModelIdLength)
+    return app_id;
+  return ShortenAppModelIdComponent(app_id, installer::kMaxAppModelIdLength);
 }
 
 base::string16 ShellUtil::BuildAppModelId(
@@ -1890,13 +1898,8 @@ base::string16 ShellUtil::BuildAppModelId(
     const base::string16& component = *it;
     DCHECK(!component.empty());
     if (component.length() > max_component_length) {
-      // Append a shortened version of this component. Cut in the middle to try
-      // to avoid losing the unique parts of this component (which are usually
-      // at the beginning or end for things like usernames and paths).
-      app_id.append(component, 0, max_component_length / 2);
-      app_id.append(component,
-                    component.length() - ((max_component_length + 1) / 2),
-                    base::string16::npos);
+      app_id.append(
+          ShortenAppModelIdComponent(component, max_component_length));
     } else {
       app_id.append(component);
     }