Remove an assert that triggers a bad cast in blink::LifecycleNotifier.

Remove an assert that triggers a bad cast in blink::LifecycleNotifier.

The bad cast happens in
blink::LifecycleNotifier<blink::ExecutionContext, blink::ContextLifecycleObserver>::context(),
when it's called from the destructor. See https://crbug.com/537398.

The primary issue is that this method makes a base-to-derived cast at the time,
when the derived instance is already destroyed.

This bug was found by Control Flow Integrity check:
https://sites.google.com/a/chromium.org/dev/developers/testing/control-flow-integrity

BUG=chromium:537398, chromium:464797
Committed: https://crrev.com/58ce4660835621899e0129250e690059d9ece8a9
Cr-Commit-Position: refs/heads/master@{#351884}

[krasin1, 2015-09-30 23:43:53]

krasin@chromium.org changed reviewers:
+ haraken@chromium.org, krasin@chromium.org

[krasin1, 2015-09-30 23:47:41]

On 2015/09/30 23:43:54, krasin1 wrote:

Hi Kentaro,

please review this CL. See https://crbug.com/537398 for more details.

Thanks!

[haraken, 2015-09-30 23:52:41]

LGTM

[krasin1, 2015-10-01 19:07:08]

The CQ bit was checked by krasin@chromium.org

[commit-bot: I haz the power, 2015-10-01 19:08:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1381823002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1381823002/1

[commit-bot: I haz the power, 2015-10-01 19:13:51]

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

[commit-bot: I haz the power, 2015-10-01 19:13:52]

Try jobs failed on following builders:
  win8_chromium_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win8_chromium_ng/...)
  win_chromium_compile_dbg_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_comp...)
  win_chromium_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)
  win_chromium_x64_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_x64_...)

[krasin, 2015-10-01 19:16:32]

The CQ bit was checked by krasin@google.com

[krasin, 2015-10-01 19:16:32]

The patchset sent to the CQ was uploaded after l-g-t-m from haraken@chromium.org
Link to the patchset: https://codereview.chromium.org/1381823002/#ps20001
(title: "no changes, just make sure the patch is accessible by bots")

[commit-bot: I haz the power, 2015-10-01 19:18:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1381823002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1381823002/20001

[commit-bot: I haz the power, 2015-10-01 21:09:09]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-01 21:10:02]

Patchset 2 (id:??) landed as
https://crrev.com/58ce4660835621899e0129250e690059d9ece8a9
Cr-Commit-Position: refs/heads/master@{#351884}