Lie to Windows about HTBOTTOMRIGHT

This is to workaround an issue where Windows can send WM_SETCURSOR and other
WM_NC* messages to the parent window, instead of the child window used by the
WebView.

Another solution would have been to enable WS_THICKFRAME on the WebView's HWND,
which turns off that peculiar Windows behavior.  However, WS_THICKFRAME might
have other unwanted side-effects.  So in this commit, it is preferable to
isolate the hack within our NC handling code.
Don't dump stack trace if AtExitManager hasn't been initialized

The stack trace code requires the AtExitManager to have been initialized.  It
it isn't initialized, it just hangs indefinitely.  This prevents the usage of
DCHECKs before the AtExitManager has been initialized.  This commit skips the
stack trace so that the app will at least abort, instead of hanging.
Remove code to support synchronous resize for in-process WebViews

A lot of this code was added in [1].  It made resizing the WebView look much
better, but the entire mechanism is too fragile and sometimes hangs while
pumping messages waiting for the backing store ack.

[1] 95c545e "Synchronize moving in-process WebViews when on the main thread"
Add modifier flags to WebView::InputEvent
Start buffering WebViewProxy::move calls again

This is a reimplementation of [1], which was superceded by [2], which was
removed in [3].  This prevents lots of move calls from getting backlogged, so
at any one point in time, there will only be one active move.

[1] 3adce1b "Buffer WebViewProxy::move() calls"
[2] 95c545e "Synchronize moving in-process WebViews when on the main thread"
[3] b2688af "Remove code to support synchronous resize for in-process WebViews"
Merge branch 'feature/blpwtk2' into release/candidate