In the age of the tech giants, start-ups face daunting odds. Through aggressive acquisitions, blatant mimicking of smaller rivals, and the built-in advantages of their user networks and internet infrastructure, tech titans rule the scene. But at least in the case of Dropbox, an online file-sharing company, it’s still possible to defy them.During its market debut last month, Dropbox’s value leapt nearly 40 percent, a jolt of optimism that arrived in the middle of a tech industry backlash.