Refactoring: Extract ExecutionContextTaskRunner from Document.

This change extracts task-executing states from Document to a new class.
This change also

- pulls some Document member functions up to ExecutionContext
- add a few ExecutionContextClient functions

so that DOMWindow-as-ExecutionContext migration becomes easier.

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

Review URL: https://codereview.chromium.org/64083002

git-svn-id: svn://svn.chromium.org/blink/trunk@161710 bbb929c8-8fbe-4397-9dbb-9b2b20218538
IDL compiler: Overloaded methods (initial)

Initial overloaded methods handling.
Overload handling is quite complicated, so I'm breaking up over several CLs.
This does basics, but does not do:
* full resolution handling (just checks length), or
* extended attributes.
Will do these in followups.

Main logic is |compute_overloads| in v8_interface.py;
template code is straightforward.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/68343002

git-svn-id: svn://svn.chromium.org/blink/trunk@161711 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove duplicated headers from the platform/ directory where the implementation file includes the same dependencies as the corresponding primary header. 

BUG=317231

Review URL: https://codereview.chromium.org/64643010

git-svn-id: svn://svn.chromium.org/blink/trunk@161712 bbb929c8-8fbe-4397-9dbb-9b2b20218538