Don't set sent_top_controls_delta when root layer doesn't exist.

Don't set sent_top_controls_delta when root layer doesn't exist.

Soon after CC initialization when the layer tree doesn't exist yet, the
top controls delta can still be set.  In that case, an early return in
LTH::ApplyScrollAndScale doesn't forward the value to Blink -- the
"sent_" value is misleading here, and this causes the delta to get
clobbered on the next commit.  Avoid this scenario by matching the
LTH-side early-return in LTHI.

NOTRY=true
BUG=423139
Committed: https://crrev.com/5d3e3b4f0fbd7a9ec4a6f41d8680e548f253797c
Cr-Commit-Position: refs/heads/master@{#299426}

[aelias_OOO_until_Jul13, 2014-10-13 23:13:50]

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

[aelias_OOO_until_Jul13, 2014-10-13 23:14:04]

Hi bokan@, PTAL.

[bokan, 2014-10-14 02:01:37]

lgtm

[aelias_OOO_until_Jul13, 2014-10-14 02:03:25]

The CQ bit was checked by aelias@chromium.org

[commit-bot: I haz the power, 2014-10-14 02:04:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/647753004/1

[commit-bot: I haz the power, 2014-10-14 02:19:13]

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

[commit-bot: I haz the power, 2014-10-14 02:19:19]

Try jobs failed on following builders:
  android_chromium_gn_compile_rel on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_chromiu...)

[aelias_OOO_until_Jul13, 2014-10-14 03:44:19]

The CQ bit was checked by aelias@chromium.org

[commit-bot: I haz the power, 2014-10-14 03:45:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/647753004/60001

[commit-bot: I haz the power, 2014-10-14 03:51:08]

Committed patchset #2 (id:60001)

[commit-bot: I haz the power, 2014-10-14 03:52:17]

Patchset 2 (id:??) landed as
https://crrev.com/5d3e3b4f0fbd7a9ec4a6f41d8680e548f253797c
Cr-Commit-Position: refs/heads/master@{#299426}