Prevent modal dialogs when preparing to swap out.

This ensures that no PageGroupLoadDeferrers will be on the stack when
processing ViewMsg_SwapOut, which would lead to an ASSERT.

BUG=312490
TEST=Start a cross-process navigation and show a loop of alerts before commit.
R=cdn@chromium.org, darin@chromium.org

Review URL: https://codereview.chromium.org/50223002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233371 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233371
Adds a class named Connector that connects a MessagePipe to a MessageReceiver.

MessageReceiver is the interface used by the generated proxies and stubs to transmit messages. Connector is responsible for reading and writing those messages on a MessagePipe. A Connector "is a" MessageReceiver so that it can accept outbound messages, and "has a" MessageReceiver so it can dispatch incoming messages.

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/54743003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233372 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233372