Fix bug in ExtractMountNameAndFullPath to set volume root as "/"

Bug found coding/testing
https://chromium-review.googlesource.com/c/chromium/src/+/1350375

Bug: 878324
Change-Id: I800bf263f17370d43c7b8413cd6af9cd819bfd0c
Reviewed-on: https://chromium-review.googlesource.com/c/1350421Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610972}

chrome/browser/chromeos/file_manager/path_util.cc

@@ -483,10 +483,13 @@ bool ExtractMountNameAndFullPath(const base::FilePath& absolute_path,
     return false;
   const std::string& value = virtual_path.value();
   size_t pos = value.find(base::FilePath::kSeparators[0]);
-  if (pos == std::string::npos)
-    return false;
   *mount_name = value.substr(0, pos);
-  *full_path = value.substr(pos);
+  // Set full_path to '/' if |absolute_path| is a root.
+  if (pos == std::string::npos) {
+    *full_path = "/";
+  } else {
+    *full_path = value.substr(pos);
+  }
   return true;
 }
 

chrome/browser/chromeos/file_manager/path_util_unittest.cc

@@ -392,6 +392,11 @@ TEST(FileManagerPathUtilTest, ExtractMountNameAndFullPath) {
   EXPECT_EQ(mount_name, downloads_mount_name);
   EXPECT_EQ(full_path, "/" + relative_path);
 
+  EXPECT_TRUE(
+      ExtractMountNameAndFullPath(downloads_path, &mount_name, &full_path));
+  EXPECT_EQ(mount_name, downloads_mount_name);
+  EXPECT_EQ(full_path, "/");
+
   EXPECT_FALSE(ExtractMountNameAndFullPath(base::FilePath("/unknown/path"),
                                            &mount_name, &full_path));
 }