Rename ScriptExecutionContext to ExecutionContext.

R=tkent@chromium.org, abarth@chromium.org, dglazkov@chromium.org
BUG=305497

Review URL: https://codereview.chromium.org/26985002

git-svn-id: svn://svn.chromium.org/blink/trunk@159430 bbb929c8-8fbe-4397-9dbb-9b2b20218538