Only call UpdateScrollOffsetDelegate once on setting delegate.

Only call UpdateScrollOffsetDelegate once on setting delegate.

This fixes a crash in virtual viewport mode caused by UpdateScrollOffsetDelegate
being called before the outer viewport's scroll delegate was set. This would
happen when SetRootLayerScrollOffsetDelegate is called, the call to the inner
scroll layer's SetScrollOffsetDelegate would call UpdateScrollOffsetDelegate
when setting the total scroll offset. This call would try to read the outer
viewport's scroll delegate causing the crash.

This CL moves the UpdateScrollOffsetDelegate from SetTotalScrollOffset to the
call sites. This allows SetRootLayerScrollOffsetDelegate to set the delegate
on both layers before updating it.

BUG=

Committed: https://crrev.com/6747f55261727d3061af4cdc4fbd9d3201ac187e
Cr-Commit-Position: refs/heads/master@{#301172}

[bokan, 2014-10-23 14:51:07]

bokan@chromium.org changed reviewers:
+ aelias@chromium.org

[bokan, 2014-10-23 14:51:07]

PTAL, this was causing crashes when running the android_webview tests with
virtual viewport on.

[aelias_OOO_until_Jul13, 2014-10-23 19:15:06]

aelias@chromium.org changed reviewers:
+ mkosiba@chromium.org

[aelias_OOO_until_Jul13, 2014-10-23 19:18:02]

This points to a problem that UpdateRootLayerState will be called once with a
bogus partial scroll offset on every activation.  This might lead to bugs later.
 Can you change it so that in pinch viewport mode, UpdateRootLayerState is
always called with the sum of the scroll offsets, or not at all?

[bokan, 2014-10-24 14:19:37]

Ok, since it's only called on setting the delegate and scrolling a layer, I
removed the UpdateScrollOffsetDelegate in SetTotalScrollOffset and moved it to
the call sites instead. I also made the call happen through a virtual on
ScrollOffsetDelegate so unit tests can override this to a no-op (to preserve
current behavior). ptal.

[mkosiba (inactive), 2014-10-24 15:54:40]

FWIW this also crashes when I try and run the WebView with virtual viewport
enabled.
lgtm

[mkosiba (inactive), 2014-10-24 16:12:51]

On 2014/10/24 15:54:40, mkosiba wrote:
> FWIW this also crashes when I try and run the WebView with virtual viewport
> enabled.

sorry, super ambiguous use of 'this'. What I wanted to say is that the crash
happens in real usage with the --enable-v..viewport flag set, not just in
unittests and that this patch seems to fix it.

[aelias_OOO_until_Jul13, 2014-10-24 18:35:27]

lgtm

[bokan, 2014-10-24 18:37:14]

The CQ bit was checked by bokan@chromium.org

[commit-bot: I haz the power, 2014-10-24 18:38:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/657063004/40001

[commit-bot: I haz the power, 2014-10-24 19:29:03]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2014-10-24 19:29:46]

Patchset 3 (id:??) landed as
https://crrev.com/6747f55261727d3061af4cdc4fbd9d3201ac187e
Cr-Commit-Position: refs/heads/master@{#301172}