Use WTF::bind instead of ExecutionContextTask in a bunch of places

This is now possible due to https://src.chromium.org/viewvc/blink?view=rev&revision=162388,
which added support for posting a Closure to ExecutionContext.

Also cleanup a bit of code, removing an unneeded static_cast in ExecutionContext.cpp
and an unused Task class in modules/webdatabase.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/80013002

git-svn-id: svn://svn.chromium.org/blink/trunk@162434 bbb929c8-8fbe-4397-9dbb-9b2b20218538