DevTools: Add checkbox for scroll bottleneck rects to settings

This change adds a checkbox to enable scroll bottleneck rects to the
rendering settings and plumbs the setting to WebLayerTreeView.

screenshot: https://docs.google.com/file/d/0B8Y78t3tjy1XaTY1RTNTSFBsY28/edit?usp=sharing

BUG=253552

Review URL: https://chromiumcodereview.appspot.com/17897002

git-svn-id: svn://svn.chromium.org/blink/trunk@153233 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: Avoid crash when quickly successively reloading a page with CSS sourcemaps

InspectorThreadableLoaderClient::didFinishLoading() can be called without
a preceding didReceiveResponse() (see CachedResource::didAddClient()),
so m_decoder may be NULL. Just leave the source empty in this case.

R=pfeldman, vsevik
BUG=255437

Review URL: https://chromiumcodereview.appspot.com/18199002

git-svn-id: svn://svn.chromium.org/blink/trunk@153234 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[CSS Grid Layout] Rename grid placement properties

The 8 May's specification changed the names of the grid positions to
grid-column-{start|end} and grid-row-{start|end}.

BUG=240317

Review URL: https://chromiumcodereview.appspot.com/17601010

git-svn-id: svn://svn.chromium.org/blink/trunk@153235 bbb929c8-8fbe-4397-9dbb-9b2b20218538