Allow child thread to install filters prior to channel connection.

Currently child thread creates and initializes the channel before adding any message filters. Since filters are added from main thread via PostTask to IO thread, it results in a race condition between messages being received and filters getting added. It is possible to miss messages if the filter is not installed in time.

This change breaks up channel initialization into two parts, creation
without connection, which allows us to add all the start up filters before
messages start getting received and connection. It also adds an option for
the subclasses of child thread to install additional filters prior to the connection.

BUG=

Committed: https://crrev.com/6f9311d4b8d822280a37d73f30c1ed141e5ecac7
Cr-Commit-Position: refs/heads/master@{#312141}

[alexst (slow to review), 2015-01-13 20:23:14]

Patchset #1 (id:1) has been deleted

[alexst (slow to review), 2015-01-15 16:08:24]

alexst@chromium.org changed reviewers:
+ piman@chromium.org

[piman, 2015-01-15 23:35:11]

LGTM, this seems reasonable. I'm not sure why we haven't run into this in, say,
the renderer which installs filters after ChildThread construction.

[alexst (slow to review), 2015-01-19 19:23:08]

On 2015/01/15 23:35:11, piman (Very slow to review) wrote:
> LGTM, this seems reasonable. I'm not sure why we haven't run into this in,
say,
> the renderer which installs filters after ChildThread construction.

Well, there are ways to keep it safe. If filtered messages don't start flowing
until an IPC is received from a renderer, for example, async filter addition is
safe because sending that message means previously posted tasks are finished.
That said, I will dig through the renderer code some more to convince myself
it's safe and open bugs if I find something racy.

[alexst (slow to review), 2015-01-19 19:23:55]

The CQ bit was checked by alexst@chromium.org

[commit-bot: I haz the power, 2015-01-19 19:24:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/847203004/20001

[commit-bot: I haz the power, 2015-01-19 20:33:26]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-01-19 20:35:00]

Patchset 1 (id:??) landed as
https://crrev.com/6f9311d4b8d822280a37d73f30c1ed141e5ecac7
Cr-Commit-Position: refs/heads/master@{#312141}