Add 'import' and 'imageset' to WebURLRequest::RequestContext enum.
Add 'import' and 'imageset' to WebURLRequest::RequestContext enum.

BUG=390497

Review URL: https://codereview.chromium.org/429213002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@179380 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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