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Priviously in the external file protocol handler, for accessing files via file
system API, we uses file system URLs having externalfile:// scheme origin
internally.  And we added special check to pass the origin in IsAccessAllowed
check.

This CL removes the special check. Instead the CL generated isolated file system
URL to access files.

BUG=367027
TEST=unit_tests --gtest_filter=*ExternalFileURL*

Review URL: https://codereview.chromium.org/615523002

Cr-Commit-Position: refs/heads/master@{#297361}