cast: webview: Update default initialization

* aura::Window default property values return 0 as the default value if
  the property isn't defined. So we can't initialize the IDs to 0 by
  default otherwise we could get matches on un-initialized Webviews.
* Register the WebviewWindowManager::Observer when the Webview is
  created since this is when we have the Webview to parent. If it
  happens that we get a parenting request without a valid Webview it
  will crash the browser process.

Bug: b/132811925
Test: Try to register Webview with ID=0
Change-Id: Ieb82c97be3850633b02894cb2a805272d38d227e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1827582


Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700293}