Revert "[jumbo] Remove 'using' declaration for mojom::blink::IDBCursor"

This reverts commit e76b26c9.

Reason for revert:
Reverting the "move content/renderer/indexed_db/ to Blink" CL.  This
commit touches a file that was created by that CL, so reverting it.
See these bugs for more info:
https://crbug.com/899446
https://crbug.com/717812

Original change's description:
> [jumbo] Remove 'using' declaration for mojom::blink::IDBCursor
> 
> The lone 'IDBCursor' can collide with the blink::IDBCursor. Remove the
> 'using' and specify the namespace instead.
> 
> TBR=pwnall@chromium.org
> 
> Bug: 717812
> Change-Id: I092b6d3c30931f45d13fd5c35af24539884fa686
> Reviewed-on: https://chromium-review.googlesource.com/c/1301434
> Commit-Queue: Fredrik Söderquist <fs@opera.com>
> Reviewed-by: Daniel Bratell <bratell@opera.com>
> Cr-Commit-Position: refs/heads/master@{#603039}

TBR=cmp@chromium.org,bratell@opera.com,fs@opera.com,pwnall@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 717812
Change-Id: I29d7de9ef16258d9cc2c6b68d5fb4ac2ad449cca
Reviewed-on: https://chromium-review.googlesource.com/c/1305245
Commit-Queue: Chase Phillips <cmp@chromium.org>
Reviewed-by: Chase Phillips <cmp@chromium.org>
Reviewed-by: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#603593}

third_party/blink/renderer/modules/indexeddb/web_idb_cursor_impl_unittest.cc

@@ -29,13 +29,14 @@ using blink::kWebIDBKeyTypeNumber;
 using blink::WebIDBValue;
 using blink::WebString;
 using blink::WebVector;
+using blink::mojom::blink::IDBCursor;
 using testing::StrictMock;
 
 namespace blink {
 
 namespace {
 
-class MockCursorImpl : public mojom::blink::IDBCursor {
+class MockCursorImpl : public IDBCursor {
  public:
   explicit MockCursorImpl(mojom::blink::IDBCursorAssociatedRequest request)
       : binding_(this, std::move(request)) {