Media Engagement: Use full paths

It looks like some of the bots are still having problems with the
preloading browser test. This ensures we use full paths for referencing
files.

BUG=794629

Change-Id: I814d74bf973968350207e4b6f4879f767c82a225
Reviewed-on: https://chromium-review.googlesource.com/827135Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524131}

chrome/browser/media/media_engagement_autoplay_browsertest.cc

@@ -142,7 +142,8 @@ class MediaEngagementAutoplayBrowserTest : public InProcessBrowserTest {
 
     // Launch the generator and wait for it to finish.
     base::CommandLine cmd(GetPythonPath());
-    cmd.AppendArg("tools/media_engagement_preload/make_dafsa.py");
+    cmd.AppendArgPath(module_dir.Append(
+        FILE_PATH_LITERAL("tools/media_engagement_preload/make_dafsa.py")));
     cmd.AppendArgPath(input_path);
     cmd.AppendArgPath(output_path);
     base::Process process = base::LaunchProcess(cmd, base::LaunchOptions());