DevTools: Support async call stacks for FileSystem API (part 1).

This patch does not yet tracks async stacks for FileWriter and FileReader.

BUG=272416, 390863
R=kinuko@chromium.org, nhiroki@chromium.org, yurys

Review URL: https://codereview.chromium.org/383123009

git-svn-id: svn://svn.chromium.org/blink/trunk@178149 bbb929c8-8fbe-4397-9dbb-9b2b20218538