Add Streams API support to XMLHttpRequest

Hidden behind the --enable-experimental-webkit-features flag.

Original Streams API spec is:
https://dvcs.w3.org/hg/streams-api/raw-file/tip/Overview.htm

How to read data from Stream in JavaScript is still under
discussion at this thread
http://lists.w3.org/Archives/Public/public-webapps/2013AprJun/0706.html
but integration with XHR is not controversial.

So, to allow APIs depending on Stream but not on JavaScript
Stream read/write API to start development (e.g. Media Source
Extensions API
https://dvcs.w3.org/hg/html-media/raw-file/tip/media-source/media-source.html),
I'd like to land XHR integration part.

BUG=169957

Review URL: https://chromiumcodereview.appspot.com/18883002

git-svn-id: svn://svn.chromium.org/blink/trunk@157609 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Kill isBlockFlow()

Loosely based off of Hyatt's patch on https://bugs.webkit.org/show_bug.cgi?id=120897

Replace calls to isBlockFlow, which returns true for BlockFlows, Grids, and
flexible boxes, with functions that are accurately named. Most callers really
just wanted isRenderBlockFlow.

Review URL: https://chromiumcodereview.appspot.com/23450035

git-svn-id: svn://svn.chromium.org/blink/trunk@157610 bbb929c8-8fbe-4397-9dbb-9b2b20218538