Fix msan error in DeepScanningDialogDelegate

See crrev.com/c/2025578 for test failure. The issue was that file_info_
was read by tests without being initialized.

Change-Id: I2c7839fc5d1aa59f07ce1a46a5ca562c6dc86da7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2024497
Commit-Queue: Dominique Fauteux-Chapleau <domfc@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736350}

chrome/browser/safe_browsing/cloud_content_scanning/deep_scanning_dialog_delegate.h

@@ -117,7 +117,7 @@ class DeepScanningDialogDelegate {
     std::string sha256;
 
     // File size in bytes. -1 represents an unknown size.
-    uint64_t size;
+    uint64_t size = 0;
   };
 
   // Callback used with ShowForWebContents() that informs caller of verdict