Rename ScriptExecutionContext to ExecutionContext.

This is reland of r159430 which was reverted as a part of r159599.
It was reverted just because of the dependency and this change itself
has no breakage.

TBR=jochen
BUG=305497

Review URL: https://codereview.chromium.org/26872003

git-svn-id: svn://svn.chromium.org/blink/trunk@159647 bbb929c8-8fbe-4397-9dbb-9b2b20218538