Use element ids as a stable identifier tracking scroll nodes across updates

Use element ids as a stable identifier tracking scroll nodes across updates

Before the pending tree is swapped to the active tree we need to record
the currently scrolling node. Once the pending tree has been pushed to
the active tree, the currently scrolling node then needs to be set.
Because the topology of the scroll tree can change, this is not as
simple as tracking the scroll node index.

Before this patch, layers were used as the stable identifier across
scroll tree updates. This patch switches to using element ids which
lets us remove another user of ScrollNode's owning_layer_id.

BUG=693740
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2723023004
Cr-Commit-Position: refs/heads/master@{#454349}
Committed: https://chromium.googlesource.com/chromium/src/+/c4e18068559bd3637c76ac46ead252b1d343098e

[pdr., 2017-03-02 00:48:20]

Description was changed from

==========
Use element ids as a stable identifier tracking scroll nodes across updates

Before the pending tree is swapped to the active tree we need to record
the currently scrolling node. Once the pending tree has been pushed to
the active tree, the currently scrolling node then needs to be set.
Because the topology of the scroll tree can change, this is not as
simple as tracking the scroll node index.

Before this patch, layers were used as the stable identifier across
scroll tree updates. This patch switches to using element ids which
lets us remove another user of ScrollNode's owning_layer_id.

BUG=693740
==========

to

==========
Use element ids as a stable identifier tracking scroll nodes across updates

Before the pending tree is swapped to the active tree we need to record
the currently scrolling node. Once the pending tree has been pushed to
the active tree, the currently scrolling node then needs to be set.
Because the topology of the scroll tree can change, this is not as
simple as tracking the scroll node index.

Before this patch, layers were used as the stable identifier across
scroll tree updates. This patch switches to using element ids which
lets us remove another user of ScrollNode's owning_layer_id.

BUG=693740
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel
==========

[pdr., 2017-03-02 00:49:01]

pdr@chromium.org changed reviewers:
+ ajuma@chromium.org

[pdr., 2017-03-02 00:49:06]

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

[commit-bot: I haz the power, 2017-03-02 00:49:47]

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, 2017-03-02 02:18:23]

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

[commit-bot: I haz the power, 2017-03-02 02:18:24]

Dry run: This issue passed the CQ dry run.

[ajuma, 2017-03-02 15:05:11]

lgtm

https://codereview.chromium.org/2723023004/diff/1/cc/trees/layer_tree_impl.cc
File cc/trees/layer_tree_impl.cc (right):

https://codereview.chromium.org/2723023004/diff/1/cc/trees/layer_tree_impl.cc...
cc/trees/layer_tree_impl.cc:441: ElementId scrolling_element_id = ElementId();
nit: no need to explicitly initialize here, we get a default-constructed
ElementId either way.

[pdr., 2017-03-02 18:21:45]

https://codereview.chromium.org/2723023004/diff/1/cc/trees/layer_tree_impl.cc
File cc/trees/layer_tree_impl.cc (right):

https://codereview.chromium.org/2723023004/diff/1/cc/trees/layer_tree_impl.cc...
cc/trees/layer_tree_impl.cc:441: ElementId scrolling_element_id = ElementId();
On 2017/03/02 at 15:05:11, ajuma wrote:
> nit: no need to explicitly initialize here, we get a default-constructed
ElementId either way.

Fixored

[pdr., 2017-03-02 18:21:46]

The CQ bit was checked by pdr@chromium.org

[pdr., 2017-03-02 18:21:47]

The patchset sent to the CQ was uploaded after l-g-t-m from ajuma@chromium.org
Link to the patchset: https://codereview.chromium.org/2723023004/#ps20001
(title: "No need for an explicit ctor")

[commit-bot: I haz the power, 2017-03-02 18:22:15]

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, 2017-03-02 19:49:34]

CQ is committing da patch.

Bot data: {"patchset_id": 20001, "attempt_start_ts": 1488478906747290,
"parent_rev": "666abc33c97400adfa53065f50364d8cbeadd89a", "commit_rev":
"c4e18068559bd3637c76ac46ead252b1d343098e"}

[commit-bot: I haz the power, 2017-03-02 19:50:12]

Description was changed from

==========
Use element ids as a stable identifier tracking scroll nodes across updates

Before the pending tree is swapped to the active tree we need to record
the currently scrolling node. Once the pending tree has been pushed to
the active tree, the currently scrolling node then needs to be set.
Because the topology of the scroll tree can change, this is not as
simple as tracking the scroll node index.

Before this patch, layers were used as the stable identifier across
scroll tree updates. This patch switches to using element ids which
lets us remove another user of ScrollNode's owning_layer_id.

BUG=693740
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel
==========

to

==========
Use element ids as a stable identifier tracking scroll nodes across updates

Before the pending tree is swapped to the active tree we need to record
the currently scrolling node. Once the pending tree has been pushed to
the active tree, the currently scrolling node then needs to be set.
Because the topology of the scroll tree can change, this is not as
simple as tracking the scroll node index.

Before this patch, layers were used as the stable identifier across
scroll tree updates. This patch switches to using element ids which
lets us remove another user of ScrollNode's owning_layer_id.

BUG=693740
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2723023004
Cr-Commit-Position: refs/heads/master@{#454349}
Committed:
https://chromium.googlesource.com/chromium/src/+/c4e18068559bd3637c76ac46ead2...
==========

[commit-bot: I haz the power, 2017-03-02 19:50:13]

Committed patchset #2 (id:20001) as
https://chromium.googlesource.com/chromium/src/+/c4e18068559bd3637c76ac46ead2...