Disable PageLoadMetricsBrowserTest.NoPaintForEmptyDocumentInChildFrame on linux

This test has recently started flaking consistently on linux. Disabling for now.
This is failing locally due to receiving valid paint updates for the main frame,
which is not expected for a seamless iframe. Indicates the problem may be coming
from blink. Test is only flaking 1/100 times, making a fix hard to verify quickly.

TBR=csharrison@chromium.org

Bug: 986642
Change-Id: Icc55950e549361169a8d07a1764c6ed06490e301
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894106Reviewed-by: John Delaney <johnidel@chromium.org>
Commit-Queue: John Delaney <johnidel@chromium.org>
Auto-Submit: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711507}

chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc

@@ -440,8 +440,8 @@ IN_PROC_BROWSER_TEST_F(PageLoadMetricsBrowserTest, NoPaintForEmptyDocument) {
                                      0);
 }
 
-// TODO(crbug.com/986642): Flaky on Win.
-#if defined(OS_WIN)
+// TODO(crbug.com/986642): Flaky on Win and Linux.
+#if defined(OS_WIN) || defined(OS_LINUX)
 #define MAYBE_NoPaintForEmptyDocumentInChildFrame \
   DISABLED_NoPaintForEmptyDocumentInChildFrame
 #else