DevTools: Async call stacks for FileSystem API part 2.

Instrumenting FileReader & FileWriter.

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

Review URL: https://codereview.chromium.org/440913002

git-svn-id: svn://svn.chromium.org/blink/trunk@179624 bbb929c8-8fbe-4397-9dbb-9b2b20218538