Fix unhandled showGuessPanel editor command

The main menu sends a -showGuessPanel: to first responder when
Edit -> Spelling and Grammar -> Show Spelling and Grammar is called.

showGuessPanel was supposed to be handled by RenderWidgetHostViewMacDelegate,
it should be forwarded to the delegate from RenderWidgetHostViewMac by
-forwardingTargetForSelector:. However, since we dynamically added
implementations for all editing commands, including showGuessPanel, to
RenderWidgetHostViewMac via RenderWidgetHostViewMacEditCommandHelper,
RenderWidgetHostViewMacDelegate didn't get to handle this command at all,
it's directly sent to WebFrameImpl::executeCommand(name, "").

Add missing handling there (previously only WebFrameImpl::executeCommand()
with command name but not value is handled) so that showGuessPanel is
correctly handled.

BUG=257627

Review URL: https://chromiumcodereview.appspot.com/18553006

git-svn-id: svn://svn.chromium.org/blink/trunk@158346 bbb929c8-8fbe-4397-9dbb-9b2b20218538