I think the idea of gp's point about others forking code means the forkers can also influence the code because they control the fork.
Otherwise, if we take your comment at face value, we'd conclude that Google's contributions to Webkit means it can't "suddenly control the direction" of Apple's Webkit. That's true, but also becomes irrelevant because Google controls its fork of "Blink".
In other words, does Apple's Webkit control us that much? No, because it seems like Google's Blink/Chromium is sufficiently independent. If we go back farther in the timeline, are we concerned that KHTML controls us? No, because it seems like Apple's fork of Webkit is independent and not beholden to KHTML decisions.
If Microsoft really wanted to, it seems like they have the resources to do what Apple did to KHTML and what Chrome did to Webkit.