Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

Currently, if a pref is changed and the pref service is enabled, the
entire value is sent to the pref service and other clients. For large
prefs used by extensions or content settings this can result
milliseconds of busy time on the UI thread.

ScopedDictionaryPrefUpdate tracks which components of a pref are changed
so only the changes need to be sent to the pref service and other pref
clients.

BUG=654988

[Sam McNally, 2017-04-11 07:00:14]

The CQ bit was checked by sammc@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-04-11 07:00:33]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-04-11 07:31:05]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-04-11 07:31:06]

Dry run: Try jobs failed on following builders:
  linux_chromium_compile_dbg_ng on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Sam McNally, 2017-04-12 00:22:20]

The CQ bit was checked by sammc@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-04-12 00:23:17]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-04-12 00:35:54]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-04-12 00:35:54]

Dry run: Try jobs failed on following builders:
  linux_chromium_tsan_rel_ng on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)
  ios-device on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-device/builds...)
  ios-simulator-xcode-clang on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-simulator-xco...)

[Sam McNally, 2017-04-12 02:56:38]

The CQ bit was checked by sammc@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-04-12 02:57:10]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-04-12 03:27:26]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-04-12 03:27:27]

Dry run: Try jobs failed on following builders:
  linux_chromium_compile_dbg_ng on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Sam McNally, 2017-04-12 04:11:59]

The CQ bit was checked by sammc@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-04-12 04:12:10]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[Sam McNally, 2017-04-12 05:17:20]

The CQ bit was checked by sammc@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-04-12 05:17:46]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-04-12 06:27:23]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-04-12 06:27:23]

Dry run: Try jobs failed on following builders:
  android_arm64_dbg_recipe on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/android_arm6...)

[Sam McNally, 2017-04-12 06:33:45]

Patchset #3 (id:40001) has been deleted

[Sam McNally, 2017-04-12 06:33:46]

Patchset #3 (id:60001) has been deleted

[Sam McNally, 2017-04-12 06:33:48]

Patchset #2 (id:20001) has been deleted

[Sam McNally, 2017-04-12 06:33:52]

Patchset #1 (id:1) has been deleted

[Sam McNally, 2017-04-12 06:34:41]

sammc@chromium.org changed reviewers:
+ tibell@chromium.org

[tibell, 2017-04-12 07:13:53]

Initial batch of comments.

https://codereview.chromium.org/2812863002/diff/80001/services/preferences/pe...
File services/preferences/persistent_pref_store_impl.cc (right):

https://codereview.chromium.org/2812863002/diff/80001/services/preferences/pe...
services/preferences/persistent_pref_store_impl.cc:139:
backing_pref_store_->SetValue(update->key, value->CreateDeepCopy(),
Can't we move out value?

https://codereview.chromium.org/2812863002/diff/80001/services/preferences/pe...
services/preferences/persistent_pref_store_impl.cc:147:
DCHECK(update->value->is_split_updates());
I'd prefer that this was in an else block without the continue statement above.
If that's too much nesting pull it out into a helper function/method.

https://codereview.chromium.org/2812863002/diff/80001/services/preferences/pu...
File services/preferences/public/cpp/lib/util.h (right):

https://codereview.chromium.org/2812863002/diff/80001/services/preferences/pu...
services/preferences/public/cpp/lib/util.h:20: void
SetValue(base::DictionaryValue* dictionary_value,
// Sets a nested value inside the dictionary, specified by |path_components|.
Creates nested dictionaries as needed.

https://codereview.chromium.org/2812863002/diff/80001/services/preferences/pu...
File services/preferences/public/cpp/scoped_pref_update.h (right):

https://codereview.chromium.org/2812863002/diff/80001/services/preferences/pu...
services/preferences/public/cpp/scoped_pref_update.h:22: // An update to a
dictionary value pref.
Could you include an example code snippet of how to use this class?

https://codereview.chromium.org/2812863002/diff/80001/services/preferences/pu...
services/preferences/public/cpp/scoped_pref_update.h:49: DictionaryValueUpdate*
value_;
Owned?

https://codereview.chromium.org/2812863002/diff/80001/services/preferences/pu...
File
services/preferences/public/cpp/tests/persistent_pref_store_client_unittest.cc
(right):

https://codereview.chromium.org/2812863002/diff/80001/services/preferences/pu...
services/preferences/public/cpp/tests/persistent_pref_store_client_unittest.cc:54:
{ ScopedDictionaryPrefUpdate update(pref_service(), kDictionaryKey); }
As per offline discussion remove these checks.

https://codereview.chromium.org/2812863002/diff/80001/services/preferences/pu...
File services/preferences/public/interfaces/preferences.mojom (right):

https://codereview.chromium.org/2812863002/diff/80001/services/preferences/pu...
services/preferences/public/interfaces/preferences.mojom:111:
array<SubPrefUpdate> split_updates;
// Updates to several values withing a pref (e.g. inside a dictionary stored
under the pref key).