Refactor ThreadableLoaderOptions for readability

ThreadableLoaderOptions shouldn't derive from ResourceLoaderOptions.
ResourceLoaderOptions members are basically just passed through to
FetchRequest while items added by ThreadableLoaderOptions definition
are configuring how CORS, etc. are handled in ThreadableLoader.
Inheritance looks making things less readable to me.

Also, I'd like to make DocumentThreadbleLoader more readable by making
constant variables held by const members.

Items in ThreadableLoaderOptions that determines ThreadableLoader's
behavior could be held by DocumentThreadableLoader as a const member.
Items defined in ResourceLoaderOptions can be altered inside
DocumentThreadableLoader, but can still be held as a const member by
having members to hold overridden items.

BUG=377541

Review URL: https://codereview.chromium.org/301243015

git-svn-id: svn://svn.chromium.org/blink/trunk@175283 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: added support for image url & node in tracing-based timeline

- add trace event for RenderImage::paintIntoRect;
- fix name of decode & resize image events for tracing vs. old timeline;
- add processing of decode/resize/paint/imageref events to TimelineTraceEventBindings;
- generalize handling of additional event properties when building details, so that
    we avoid extra switches over event type by extracting properties relevant for
    details into TraceEvent properties;
- temporary workaround for crash on record stop in tests by cloning an AtomicString
    (see crbug.com/375242 for more details).

BUG=361045,375242

Review URL: https://codereview.chromium.org/308123008

git-svn-id: svn://svn.chromium.org/blink/trunk@175284 bbb929c8-8fbe-4397-9dbb-9b2b20218538