Reduce memory peaks when generating data urls for images.

Adding Vector<char> to a String results in one (or more) temporary
copies of that Vector<char> since Vector isn't a pointer/smart pointer
like the types usually used.

Luckily there are many other ways to solve the problem. With StringBuilder
or using a String.

BUG=389085 (related to, not necessarily fixing it)

Review URL: https://codereview.chromium.org/559103002

git-svn-id: svn://svn.chromium.org/blink/trunk@181925 bbb929c8-8fbe-4397-9dbb-9b2b20218538