Couple of minor changes to the WebKitAPI. This is a small step in a larger work-in-progress. See the bug for details.

* Distinguish more clearly between blobURLs and fileSystemURLs in WebBlobData and the very similar WebHTTPBody element structs.

* Put in place some of the new API to identify blob data by uuid instead of blob instances by URL. What's in place enough for Blink to call out to Chromium using the new id scheme. What's not in place is enough for Chrome to call into Blink using the new scheme as Blink doesn't know how to do that yet. Blink doesn't use the new API yet but Chromium is nearly ready for it to start doing so after https://codereview.chromium.org/23223003/ lands.

BUG=174200

Review URL: https://chromiumcodereview.appspot.com/23703003

git-svn-id: svn://svn.chromium.org/blink/trunk@157024 bbb929c8-8fbe-4397-9dbb-9b2b20218538