Reland "Ensure child_path for SetProjectId() is a relative path"

This reverts commit 43cd4f53.

This time, I removed DCHECK and also inlined
FilePath("/").AppendRelativePath() directly in the caller.

BUG=b:171847983
TEST=unittest

Change-Id: I4ff3ed1313a0eb38457b8b1d2c1ca1ddf450473f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612789Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Satoshi Niwa <niwa@chromium.org>
Auto-Submit: Satoshi Niwa <niwa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840975}

components/arc/disk_quota/arc_disk_quota_bridge.cc

@@ -57,27 +57,29 @@ bool ArcDiskQuotaBridge::convertPathForSetProjectId(
     return false;
   }
 
+  *child_path_out = base::FilePath();
   if (kDownloadPath.IsParent(android_path)) {
     // /storage/emulated/0/Download/* =>
     //     parent=/home/user/<hash>/Downloads/, child=*
     *parent_path_out = cryptohome::SetProjectIdAllowedPathType::PATH_DOWNLOADS;
-    *child_path_out = base::FilePath();
     return kDownloadPath.AppendRelativePath(android_path, child_path_out);
   } else if (kExternalStoragePath.IsParent(android_path)) {
     // /storage/emulated/0/* =>
-    //     parent=/home/root/<hash>/android-data/, chile=/data/media/0/*
+    //     parent=/home/root/<hash>/android-data/, child=data/media/0/*
     *parent_path_out =
         cryptohome::SetProjectIdAllowedPathType::PATH_ANDROID_DATA;
-    *child_path_out = kDataMediaPath;
-    return kExternalStoragePath.AppendRelativePath(android_path,
+    // child_path should be relative to the root.
+    return base::FilePath("/").AppendRelativePath(kDataMediaPath,
+                                                  child_path_out) &&
+           kExternalStoragePath.AppendRelativePath(android_path,
                                                    child_path_out);
   } else if (kDataMediaPath.IsParent(android_path)) {
     // /data/media/0/* =>
-    //     parent=/home/root/<hash>/android-data/, child=/data/media/0/*
+    //     parent=/home/root/<hash>/android-data/, child=data/media/0/*
     *parent_path_out =
         cryptohome::SetProjectIdAllowedPathType::PATH_ANDROID_DATA;
-    *child_path_out = android_path;
-    return true;
+    // child_path should be relative to the root.
+    return base::FilePath("/").AppendRelativePath(android_path, child_path_out);
   } else {
     return false;
   }

components/arc/disk_quota/arc_disk_quota_bridge_unittest.cc

@@ -26,13 +26,13 @@ TEST(ArcDiskQuotaBridgeTest, ConvertPathForSetProjectId) {
       base::FilePath("/storage/emulated/0/Pictures/test.png"), &parent_path,
       &child_path));
   EXPECT_EQ(PathType::PATH_ANDROID_DATA, parent_path);
-  EXPECT_EQ("/data/media/0/Pictures/test.png", child_path.value());
+  EXPECT_EQ("data/media/0/Pictures/test.png", child_path.value());
 
   EXPECT_TRUE(ArcDiskQuotaBridge::convertPathForSetProjectId(
       base::FilePath("/data/media/0/Movies/test.mp4"), &parent_path,
       &child_path));
   EXPECT_EQ(PathType::PATH_ANDROID_DATA, parent_path);
-  EXPECT_EQ("/data/media/0/Movies/test.mp4", child_path.value());
+  EXPECT_EQ("data/media/0/Movies/test.mp4", child_path.value());
 
   // Unallowed path.
   EXPECT_FALSE(ArcDiskQuotaBridge::convertPathForSetProjectId(