“distrust” is an established English words that means exactly what Google intends. “detrust” is, while not hard to figure out, an unnecessary neologism.
“distrust” alone only communicates not trusting, true. But the word isn't alone, it is in the phrase “plan to distrust”, which communicates that the not-trusting is a change from the status quo that will occur in the future.
Not necessarily. "Plan to distrust" would be consistent with "a company has announced that they're going to start issuing certificates; we won't trust them". You and I know that Symantec is a major CA which is currently trusted by Chrome, but that isn't implied by the headline.
http://legaltimes.typepad.com/files/garner-transcripts-1.pdf