Previously thumbnail requests for context nodes were done with two IPC
messages: ChromeViewMsg_RequestThumbnailForContextNode &
ChromeViewHostMsg_RequestThumbnailForContextNode_ACK.

This CL introduces a new per-frame mojo interface, ThumbnailCapturer,
that's exposed by the renderer to the browser.

BUG=699368
TEST=existing browsertest, manually tested on linux, mac, android

Review-Url: https://codereview.chromium.org/2737893002
Cr-Commit-Position: refs/heads/master@{#456632}