Drop isindex from HTMLTagNames.

Followup from https://codereview.chromium.org/96653004/

This requires an update to the generation scripts, as they must treat
the fallback interface as an interface supporting multiple tag names.
This only accidentally worked, as both 'image' and 'isindex' were
listed as using the HTMLUnknownElement interface. Removing 'isindex'
exposed the bug.

"Intent to Remove" discussion: https://groups.google.com/a/chromium.org/d/msg/blink-dev/14q_I06gwg8/0a3JI0kjbC0J

Review URL: https://codereview.chromium.org/182843002

git-svn-id: svn://svn.chromium.org/blink/trunk@168033 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix audits severity bullet + add audits main element width constraint.

BUG=347060

Review URL: https://codereview.chromium.org/176893004

git-svn-id: svn://svn.chromium.org/blink/trunk@168035 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Revert of Turn window.postMessage into an ActiveDOMObject (https://codereview.chromium.org/157243003/)

Reason for revert:
A tentative cause for crashes of
fast/dom/custom/document-register-reentrant-throwing-constructor.html,
fast/events/before-unload-return-bad-value.html,
fast/events/onerror-no-constructor.html

Original issue's description:
> Turn window.postMessage into an ActiveDOMObject
> 
> When JavaScript execution is paused by the debugger messages sent via postMessage should be suspended similar to other events.
> 
> BUG=224874
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168026

TBR=pfeldman@chromium.org,aandrey@chromium.org,abarth@chromium.org,atwilson@chromium.org,yurys@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=224874

Review URL: https://codereview.chromium.org/183293002

git-svn-id: svn://svn.chromium.org/blink/trunk@168036 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Have ElementData::attributeItem() return a reference

Have ElementData::attributeItem() return a reference instead of a pointer as it
can never return null. This makes the calling code look safer.

R=adamk, eseidel

Review URL: https://codereview.chromium.org/180723006

git-svn-id: svn://svn.chromium.org/blink/trunk@168037 bbb929c8-8fbe-4397-9dbb-9b2b20218538