Upgrade Blink to milliseconds-based last modified filetimes, part 4.

Have content::FileInfoToWebFileInfo() set the temporarily Blink-unused
field WebFileInfo::modificationTime to something milliseconds-based

Once landed, this is to allow Blink to switch to no longer using
WebFileInfo::modificationTimeMS, but modificationTime only (again.)

This is the fourth patch in the following series,

1: [blink] add WebFileInfo::modificationTimeMS [ https://codereview.chromium.org/873723004/ ]
2: [chromium] fill in modificationTimeMS [ https://codereview.chromium.org/884413002/ ]
3: [blink] make use of modificationTimeMS [ https://codereview.chromium.org/884393002/ ]
4: [chromium] *this one* [ https://codereview.chromium.org/862203003/ ]
5: [blink] switch to using modificationTime instead of *MS [ https://codereview.chromium.org/882343002/ ]
6: [chromium] stop setting modificationTimeMS [ https://codereview.chromium.org/890523002/ ]
7: [blink] remove modificationTimeMS [ https://codereview.chromium.org/869613005/ ]

R=jochen
BUG=451747

Review URL: https://codereview.chromium.org/862203003

Cr-Commit-Position: refs/heads/master@{#315322}

content/child/file_info_util.cc

@@ -12,13 +12,13 @@ namespace content {
 void FileInfoToWebFileInfo(const base::File::Info& file_info,
                            blink::WebFileInfo* web_file_info) {
   DCHECK(web_file_info);
-  // WebKit now expects NaN as uninitialized/null Date.
+  // Blink now expects NaN as uninitialized/null Date.
   if (file_info.last_modified.is_null()) {
     web_file_info->modificationTime = std::numeric_limits<double>::quiet_NaN();
     web_file_info->modificationTimeMS =
         std::numeric_limits<double>::quiet_NaN();
   } else {
-    web_file_info->modificationTime = file_info.last_modified.ToDoubleT();
+    web_file_info->modificationTime = file_info.last_modified.ToJsTime();
     web_file_info->modificationTimeMS = file_info.last_modified.ToJsTime();
   }
   web_file_info->length = file_info.size;