Fix -Wdefaulted-function-deleted warning

This is part of cleaning up for a new Clang warning that's currently disabled:

../../chrome/chrome_cleaner/scanner/urza_scanner_impl_unittest.cc(121,3):
error: explicitly defaulted default constructor is implicitly deleted
[-Werror,-Wdefaulted-function-deleted]
  TestScanner() = default;
  ^
../../chrome/chrome_cleaner/scanner/urza_scanner_impl_unittest.cc(119,21):
note: default constructor of 'TestScanner' is implicitly deleted because base
class 'chrome_cleaner::UrzaScannerImpl' has no default constructor
class TestScanner : public UrzaScannerImpl {
                    ^

Bug: 890307
Change-Id: I0ae013e278b85ae3ea706b1ecb716d5382a32132
Reviewed-on: https://chromium-review.googlesource.com/c/1340316
Commit-Queue: Joe Mason <joenotcharles@chromium.org>
Reviewed-by: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608996}

chrome/chrome_cleaner/scanner/urza_scanner_impl_unittest.cc

@@ -118,7 +118,6 @@ void set_difference(const std::set<T>& set1,
 
 class TestScanner : public UrzaScannerImpl {
  public:
-  TestScanner() = default;
   explicit TestScanner(const MatchingOptions& options,
                        SignatureMatcherAPI* signature_matcher,
                        RegistryLogger* registry_logger)