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The Webview WebContents and the aura::Window it attaches to are not
created in the same place and there is a disconnect between how the EXO
window is created and how the Webview is created.

Webview creation event results in a request to create an EXO window.
Thus WebviewRpcInstance doesn't have a window to display in until the
client creates an EXO window with the appropriate ID. There needs
to be a way to listen for window events to discover the EXO window
hosting the Webview. WebviewWindowManager listens for new
aura::Windows to be created and registers as an observer on them to
find windows with the exo::kClientSurfaceIdKey property. The ID is then
used to find the Webview that will be attached to the window.

BUG=b/132811925
TEST=Compiled

Change-Id: I053e2b792b311f0c95e791ef5c70a9e18b24f5ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1803957
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696906}