Add Java wrapper for RenderFrameHost

This patch adds a Java wrapper for RenderFrameHost. It also adds a
getMainFrame method to WebContents.

This will be useful for Payment Request which needs to know
the exact (sub)frame issued the payment request.

BUG=620173
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2681933002
Cr-Commit-Position: refs/heads/master@{#455635}
19 files changed