Update documentation for chrome/test/base/in_process_browser_test.h.

The file does not provide the preprocessor defines for
IN_PROC_BROWSER_TEST_ macros. This documentation update makes it clear
to users that they need to manually include
"content/public/test/browser_test.h". The header cannot be directly
included by in_process_browser_test.h as it produces compile errors:

https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8874211071355292448/+/steps/compile__with_patch_/0/stdout

Change-Id: Ie8091857d76e2408a6fdbc471bf97ca544f8c89d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2308863Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790530}

chrome/test/base/in_process_browser_test.h

@@ -69,6 +69,10 @@ class ScopedBundleSwizzlerMac;
 //   SetUpOnMainThread(), SetUpInProcessBrowserTestFixture(), and other related
 //   methods for a cleaner alternative).
 //
+// To include the default implementation of RunTestOnMainThread() and TestBody()
+// for Gtests, it's also necessary to include the file
+// "content/public/test/browser_test.h"
+//
 // The following hook methods are called in sequence before BrowserMain(), so
 // no browser has been created yet. They are mainly for setting up the
 // environment for running the browser.