In a calm and understated blog post, Google said Tuesday that it had recently come under an unusual cyberattack from China. The Web giant said the attack was very different from previous ones because it was aimed at Chinese dissidents’ Gmail accounts.... Information on two accounts was accessed by the hackers, Google said. In addition, “the accounts of dozens of U.S.-, China- and Europe-based Gmail users who are advocates of human rights in China appear to have been routinely accessed by third parties…most likely via phishing scams or malware placed on the users’ computers,” the company wrote. ... Google said it had identified about 20 other companies that had been similarly targeted.