Added trace event to Resource::appendData. This is part of an effort to identify what contributes to the ~12.6% "uncategorized" time from the android top 25k experiment. For sites high in uncategorized time, this accounts for about 0.7% of the trace.

BUG=

Review URL: https://codereview.chromium.org/26908004

git-svn-id: svn://svn.chromium.org/blink/trunk@159456 bbb929c8-8fbe-4397-9dbb-9b2b20218538