Add an isClickable attribute to DOMSnapshot DOMNode

Bug: 
Change-Id: I8c5d4fdc8ecd2cf8fe66230c872aa75791967e5d
Reviewed-on: https://chromium-review.googlesource.com/553141
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#483704}

• headless/BUILD.gn[diff]
• headless/lib/dom_tree_extraction_expected_nodes.txt[Added - diff]
• headless/lib/dom_tree_extraction_expected_styles.txt[Added - diff]
• headless/lib/headless_devtools_client_browsertest.cc[diff]
• third_party/WebKit/LayoutTests/inspector-protocol/dom-snapshot/dom-snapshot-getSnapshot-expected.txt[diff]
• third_party/WebKit/LayoutTests/inspector-protocol/resources/dom-snapshot.html[diff]
• third_party/WebKit/Source/core/inspector/InspectorDOMSnapshotAgent.cpp[diff]
• third_party/WebKit/Source/core/inspector/browser_protocol.json[diff]