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Extensions with background pages (e.g. Google Cast) currently have a
shorter code path when showing an extension popup that can cause the
renderer to ask for screen metrics before the hosting NSView is added to
the view hierarchy. Since the NSWindow is nil in this case, screen
metrics for the primary screen are used, and they may be the incorrect
screen for the popup.

On non-Mac, toolkit-views platforms get around this problem by deferring
creation until View::ViewHierarchyChanged is triggered. However, there
is no NSView apart from the render view on the extension-side on Mac, so
to do the same we'd either need to add one (and keep it sized
appropriately), or modify the WebContents view itself to override
[NSView viewDid/WillMoveToSuperview] and feed it through to embedders.

Instead, this CL tweaks the ExtensionPopupController initialization to
always create the popup NSWindow (initially hidden), before creating the
ExtensionViewHost.

BUG=324748, 305620
TEST=On a retina mac, plug in a (non-retina) external monitor, move
Chrome there, and open the Chromecast dialog. It should look "nice" (not
as nice as the retina screen, but consistent with other text in Chrome).
See http://crbug.com/324748#c41

Review URL: https://codereview.chromium.org/653843002

Cr-Commit-Position: refs/heads/master@{#317011}