Make MHTMLGenerationSitePerProcessTest run.

https://crrev.com/c/1500020 made this test parameterized but did not
add INSTANTIATE_TEST_SUITE_P(MHTMLGenerationSitePerProcessTest).
Googletest warns about this in its console output but doesn't fail.

I've filed https://crbug.com/1045846 to try prevent this happening in
future, as I almost did something similar.

Change-Id: I6b08f79e007612f3c58c54b8554401f48bf48a76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2022564Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735640}

content/browser/download/mhtml_generation_browsertest.cc

@@ -941,5 +941,8 @@ IN_PROC_BROWSER_TEST_F(MHTMLGenerationTest, GenerateMHTMLButDelayResponse) {
 INSTANTIATE_TEST_SUITE_P(MHTMLGenerationTest,
                          MHTMLGenerationTest,
                          testing::Combine(testing::Bool(), testing::Bool()));
+INSTANTIATE_TEST_SUITE_P(MHTMLGenerationSitePerProcessTest,
+                         MHTMLGenerationSitePerProcessTest,
+                         testing::Combine(testing::Bool(), testing::Bool()));
 
 }  // namespace content