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HostZoomMapObserver is a per-WebContents class, while the HostZoom Mojo
connection is per-frame. Before this CL, HostZoomMapObserver was
maintaining one HostZoom connection and rebinding it every time a new
RenderFrame was created. This meant that HostZoomMapObserver was
continually losing connections to existing frames. This CL changes
HostZoomMapObserver to maintain one connection per-RenderFrame in a
map indexed by the corresponding RenderFrameHost.

BUG=673065
TEST=Visit news.ycombinator.com and increase the zoom level to 175%.
Click the top link. Hit back: news.ycombinator.com should still be
zoomed to 175%.

Review-Url: https://codereview.chromium.org/2581143002
Cr-Commit-Position: refs/heads/master@{#439800}