pdf-viewer/mandoline: Create new instances of tree-delegate/frame-client for each view.
pdf-viewer/mandoline: Create new instances of tree-delegate/frame-client for each view.

. The ContentHandler implementation creates an instance of
  PDFViewerApplicationDelegate for each document.

. The PDFViewerApplicationDelegate opens the PDF document, and is responsible
  for creating all the views for displaying the document. It creates a new
  instance of PDFView for each view.

. PDFView is responsible for managing a single view displaying a pdf document.
  It provides the mus::WindowTreeDelegate and web_view::FrameClient
  implementations for each view.

. The PDFView notifies the PDFViewerApplicationDelegate about termination
  through the PDFViewDelegate interface. When all views for a document are
  terminated, PDFViewerApplicationDelegate closes the PDF document, and tears
  down the app.

BUG=544997

Review URL: https://codereview.chromium.org/1414083002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#355469}

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