Important Sites: swap std::string fill constructor arguments.

The constructor's argument order was flipped. Please see the "fill(6)"
constructor for more info: http://www.cplusplus.com/reference/string/string/string/

Previously, this would cast ' ' to a size_t, and length to a char.

Change-Id: I6eca18bfa48fa3f303ea2881f3ebfd342e72742c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2432747
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Auto-Submit: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811080}

chrome/browser/engagement/important_sites_usage_counter_unittest.cc

@@ -75,7 +75,7 @@ class ImportantSitesUsageCounterTest : public testing::Test {
         profile()->GetPath().AppendASCII("Local Storage");
     base::CreateDirectory(storage_path);
 
-    std::string data(' ', length);
+    std::string data(length, ' ');
     // Write file to local storage.
     base::FilePath file_path = storage_path.Append(storage_origin);
     base::WriteFile(file_path, data.c_str(), length);