Fix Tab.getWebContents for downstream instrumentation tests

Fix Tab.getWebContents for downstream instrumentation tests

Tab.getWebContents() was using JNI to fetch the WebContents object. This
is undesirable for several reasons, including performance impact and
thread access consistency. Remove this and prefer redirecting the query
to ContentViewcore.getWebContents() instead.

This fix is a temporary workaround until all downstream test sites can
be patched.

BUG=398263, 414445
TBR=tedchoc@chromium.org
NOTRY=true

Committed: https://crrev.com/9013fc8939168ed393119ce440e2630b3b68ed1c
Cr-Commit-Position: refs/heads/master@{#294918}

[jdduke (slow), 2014-09-15 21:25:11]

jdduke@chromium.org changed reviewers:
+ tedchoc@chromium.org

[jdduke (slow), 2014-09-15 21:25:45]

tedchoc@: I'm not sure we can do much better than this for now... I'd like to
add some ThreadUtils assertions, but we'll need to patch downstream tests first.

[jdduke (slow), 2014-09-15 21:25:52]

The CQ bit was checked by jdduke@chromium.org

[commit-bot: I haz the power, 2014-09-15 21:27:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/571813003/1

[jdduke (slow), 2014-09-15 22:08:31]

The CQ bit was unchecked by jdduke@chromium.org

[jdduke (slow), 2014-09-15 22:18:08]

The CQ bit was checked by jdduke@chromium.org

[Ted C, 2014-09-15 22:36:49]

lgtm

[commit-bot: I haz the power, 2014-09-15 22:39:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/571813003/20001

[commit-bot: I haz the power, 2014-09-15 23:29:33]

Committed patchset #2 (id:20001) as 28ecc42dfed8a63c30f405a2434ad76b866433f0

[commit-bot: I haz the power, 2014-09-15 23:32:22]

Patchset 2 (id:??) landed as
https://crrev.com/9013fc8939168ed393119ce440e2630b3b68ed1c
Cr-Commit-Position: refs/heads/master@{#294918}