DevTools: fix backing storage offset computation for trace events

We erroneously used charaters, not bytes as offsets into trace events
backing storage, which cuased problems when non-ascii characters appeared
in trace. Use blobs to early convert to utf8 and get the correct byte
size.

BUG=

Review URL: https://codereview.chromium.org/657113005

git-svn-id: svn://svn.chromium.org/blink/trunk@184460 bbb929c8-8fbe-4397-9dbb-9b2b20218538