Check for nullptr in GetInnerWebContents()

Not all platforms provide a BrowserPluginGuestManager (e.g. Cast), so
perform a nullptr check before using it.

Bug: internal b/132814855
Test: CQ
Change-Id: I819dc85c214e292ae72068c7b8d48e266209a938
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1613972
Reviewed-by: Avi Drissman <[email protected]>
Reviewed-by: Lucas Gadani <[email protected]>
Reviewed-by: Charlie Reis <[email protected]>
Commit-Queue: Sean Topping <[email protected]>
Cr-Commit-Position: refs/heads/master@{#660449}
1 file changed