[blink] Show spelling suggestions when pressing context menu key.

The EventHandler::sendContextMenuEventForKey() handler does not invoke
context-menu hooks. This results in absence of spellcheck functionality in
the context menu brought up with a context-menu keyboard key. (Spellcheck
functionality is present only in the context menu brought up with a right
mouse button click.) The fix is to do the same thing as what the 
EventHandler::sendContextMenuEventForGesture() handler does: replacing
dispatchMouseEvent() call with handleMousePressEvent() and 
sendContextMenuEvent() calls. Note that sendContextMenuEvent() will invoke
dispatchMouseEvent().

Old flow:

sendContextMenuEventForKey
  -> dispatchMouseEvent

New flow:

sendContextMenuEventForKey
  -> handleMousePressEvent, sendContextMenuEvent
    -> dispatchMouseEvent

BUG=282598

Review URL: https://codereview.chromium.org/123583002

git-svn-id: svn://svn.chromium.org/blink/trunk@167888 bbb929c8-8fbe-4397-9dbb-9b2b20218538