Make GC test catch strong ref from client objects

Make GC test catch strong ref from client objects

In the public sdk API, apps should be able to hold a strong WebView
reference from WebChromeClient or WebViewClient, without having to worry
about preventing WebView from gc-ed.

Simulate that behavior in testCreateAndGcManyTimes by having the
AwContentsClient hold a strong reference to AwContents.

And remove the strong ref from AwWebContentsObserver to
make the test pass.

BUG=469803
Committed: https://crrev.com/9a424d2eeb702d73fb6173e3e70827d77d68ddb7
Cr-Commit-Position: refs/heads/master@{#322380}

[boliu, 2015-03-25 19:05:11]

boliu@chromium.org changed reviewers:
+ torne@chromium.org

[boliu, 2015-03-25 19:05:11]

torne for review

We might want to do an audit of all our callback interfaces. VisualStateCallback
already kinda breaks this, but that callback is guaranteed to clear at least.

[Torne, 2015-03-26 10:31:07]

lgtm

[boliu, 2015-03-26 13:53:21]

The CQ bit was checked by boliu@chromium.org

[commit-bot: I haz the power, 2015-03-26 13:53:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1036893002/40001

[boliu, 2015-03-26 14:17:20]

The CQ bit was unchecked by boliu@chromium.org

[boliu, 2015-03-26 14:22:39]

Had to suppress an "Unread field" findbugs error in the
StrongRefTestAwContentsClient class.

[boliu, 2015-03-26 14:22:43]

The CQ bit was checked by boliu@chromium.org

[boliu, 2015-03-26 14:22:43]

The patchset sent to the CQ was uploaded after l-g-t-m from torne@chromium.org
Link to the patchset: https://codereview.chromium.org/1036893002/#ps60001
(title: "findbugs")

[commit-bot: I haz the power, 2015-03-26 14:23:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1036893002/60001

[commit-bot: I haz the power, 2015-03-26 14:53:03]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-03-26 14:53:59]

Patchset 4 (id:??) landed as
https://crrev.com/9a424d2eeb702d73fb6173e3e70827d77d68ddb7
Cr-Commit-Position: refs/heads/master@{#322380}