[DevTools] Address crash by checking if Tab loaded successfully

[DevTools] Address crash by checking if Tab loaded successfully

* Changes Tab::LoadIfNeeded()/Tab.loadIfNeeded() to return true
  if the call was "handled" and false if it failed.

* DevTools checks if LoadIfNeeded() fails, and bails out if it
  does.

BUG=403592
Committed: https://crrev.com/b90614c44a566be2ed618df1e036cd441538c6ef
Cr-Commit-Position: refs/heads/master@{#297482}

R=dtrainor@chromium.org

Committed: https://chromium.googlesource.com/chromium/src/+/251bb226ba998ce3b25f7a10116600d124c57a69

[dgozman, 2014-09-30 11:55:04]

dgozman@chromium.org changed reviewers:
+ dgozman@chromium.org

[dgozman, 2014-09-30 11:55:05]

I support this change.

[gone, 2014-09-30 18:08:57]

dfalcantara@chromium.org changed reviewers:
+ dtrainor@chromium.org

[David Trainor- moved to gerrit, 2014-09-30 18:34:20]

lgtm

[gone, 2014-09-30 18:35:45]

The CQ bit was checked by dfalcantara@chromium.org

[commit-bot: I haz the power, 2014-09-30 18:37:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/618513002/1

[commit-bot: I haz the power, 2014-09-30 19:34:26]

Committed patchset #1 (id:1) as 2ea66f5ad2f30dcad7ffa534c34d24f4e0058a34

[commit-bot: I haz the power, 2014-09-30 19:35:04]

Patchset 1 (id:??) landed as
https://crrev.com/b90614c44a566be2ed618df1e036cd441538c6ef
Cr-Commit-Position: refs/heads/master@{#297482}

[gone, 2014-10-03 17:45:34]

Committed patchset #1 (id:1) manually as
251bb226ba998ce3b25f7a10116600d124c57a69 (presubmit successful).