Add an IPC channel between the NaCl loader process and the renderer.

Add an IPC channel between the NaCl loader process and the renderer.
BUG=116317
TEST=ppapi, nacl tests, manual testing for experimental IPC proxy.
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=143656

[bbudge, 2012-06-22 15:17:40]

This uses a global map to store ChannelHandles per PP_Instance as before.
There's no way to prevent leaks if some subsequent error causes the Plugin to
not Start the PPAPI proxy.  PTAL

[bbudge, 2012-06-22 16:56:56]

Some background: The previous version of this CL broke NaCl integration,
specifically two tests of failure behavior. The problem is that passing a void*
back to the plugin guarantees a leak if the proxy isn't started. The last CL
tried to delete this but it can't be done safely.

Mark suggested combining LaunchSelLdr and StartPpapiProxy into a single call,
sidestepping the need to pass anything back to the Plugin. Unfortunately it
doesn't seem possible to merge the two calls. The problem is that ServiceRuntime
doesn't initialize communications with the sel_ldr until after LaunchSelLdr
returns. If I try to start the proxy in LaunchSelLdr, it starts the IRT, which
then hangs the sel_ldr, because the SRPC channel isn't ready.

This CL goes back to keeping a global map to associate PP_Instances and
ChannelHandles. Successful startups will leave the map empty. Unsuccessful ones
will leave the ChannelHandle in the map. We should be able to clean them up by
using the PluginModule to tell us which are live instances, or by clearing the
map once we have the process shutdown event working.

[brettw, 2012-06-22 17:56:30]

lgtm

[dmichael (off chromium), 2012-06-22 18:05:06]

lgtm