Fix missing sidebar in Chrome 34 and later

See #29.

I updated blink-devtools to the latest (requires sebmarkbage/blink-devtools#1)
and did a manual merge, changing ReactPanel.js to match all changes that
happened to ElementPanel.js since the last devtools version.

I'm not sure we actually want to continue tracking the latest devtools; we
could presumably keep an old version of the panels and just fix the specific
problem with the removed SidebarView.

This doesn't actually work properly yet due to some style-related bug; when
the panel is rendered, the View components read all of their dimensions as 0,
then 0 is set as the actual width and height of each element, causing nothing
to display. However, if the script debugger is used within `ReactPanel.show`
to inspect `this.element` (but not to change any properties on it), then the
panel seems to display correctly. I'm not sure how to go about fixing this. I
can look more later, but if anyone has ideas here I'd appreciate it.