Introduce ChromeZoomLevelPref, make zoom level prefs independent of profile prefs.

chrome/browser/profiles/profile.cc

Index: chrome/browser/profiles/profile.cc
diff --git a/chrome/browser/profiles/profile.cc b/chrome/browser/profiles/profile.cc
index 86326aa08254a89069e62c9a11413e3bb6e9d9d3..f2e29b3d7690643dd7b2eba07f7499044bd6650d 100644
--- a/chrome/browser/profiles/profile.cc
+++ b/chrome/browser/profiles/profile.cc
@@ -60,6 +60,10 @@ TestingProfile* Profile::AsTestingProfile() {
   return NULL;
 }
 
+chrome::ChromeZoomLevelPrefs* Profile::GetZoomLevelPrefs() {
+  return NULL;
+}
+
 Profile::Delegate::~Delegate() {
 }
 
@@ -136,11 +140,27 @@ void Profile::RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry) {
       prefs::kSelectFileLastDirectory,
       std::string(),
       user_prefs::PrefRegistrySyncable::UNSYNCABLE_PREF);
+  // TODO(wjmaclean): remove the following pref once migration to per-partition
+  // zoom is complete.
   registry->RegisterDoublePref(
-      prefs::kDefaultZoomLevel,
+      prefs::kProfileDefaultZoomLevel,

[Bernhard Bauer, 2014/10/01 15:09:14]

This is what I meant in my email: You're registering a preference which is a
prefix of another preference. That is very dangerous and might break at any
time. For example, someone could write a dictionary to
profile.default_zoom_level which doesn't contain a double for the key "0". Then
when you're trying to read profile.default_zoom_level.0, you'll get back the
wrong type, which will DCHECK. I think you'll need to implement the type
checking and default value logic anyway for arbitrary subdictionaries under
profile.default_zoom_level, so registering profile.default_zoom_level.0 doesn't
get you anything you couldn't get otherwise.

[wjmaclean, 2014/10/01 20:32:59]

I assume this comment was intended for line 151 (below) ... this line of code is
part of the legacy prefs for default zoom level, with a re-name of the constant
(prefs::kProfileDefaultZoomLevel).

But I understand that I cannot register both prefs::kDefaultZoomLevel and
prefs::kDefaultZoomLevel + ".0".

The main reason that I wanted to register partition.default_zoom_level.0 was to
get notifications if somewhere else in the code modifies this value (this is the
default zoom level for the main browser window). So *not* registering this means
I will have to add custom notifications for this value.

+      0.0,
+      user_prefs::PrefRegistrySyncable::UNSYNCABLE_PREF);
+  std::string default_zoom_level_path(prefs::kDefaultZoomLevel);
+  default_zoom_level_path.append(".0");
+  registry->RegisterDoublePref(
+      default_zoom_level_path.c_str(),
       0.0,
       user_prefs::PrefRegistrySyncable::UNSYNCABLE_PREF);
   registry->RegisterDictionaryPref(
+      prefs::kDefaultZoomLevel,
+      user_prefs::PrefRegistrySyncable::UNSYNCABLE_PREF);
+  // TODO(wjmaclean): remove the following pref once migration to per-partition
+  // zoom is complete.
+  registry->RegisterDictionaryPref(
+      prefs::kProfilePerHostZoomLevels,
+      user_prefs::PrefRegistrySyncable::UNSYNCABLE_PREF);
+  registry->RegisterDictionaryPref(
       prefs::kPerHostZoomLevels,
       user_prefs::PrefRegistrySyncable::UNSYNCABLE_PREF);
   registry->RegisterStringPref(