Thumbnail capturing: Increase sleep time in SlowPageLoad test

The test has some flakes on Win dbg, which is likely just a bit too
slow. This CL increases the sleep time from 100 to 200 ms to hopefully
resolve the flakes.

TBRing this single-character test-only change, so it can accumulate
some runs over the weekend.
TBR=mastiz

Bug: 778248
Change-Id: I9c9d0a39023d76ad4bea05aa8f700bd310d4f284
Reviewed-on: https://chromium-review.googlesource.com/776660Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517455}

chrome/browser/thumbnails/thumbnail_browsertest.cc

@@ -334,7 +334,7 @@ IN_PROC_BROWSER_TEST_F(ThumbnailTest,
   // Give the renderer process some time to actually paint it. Without this,
   // there's a chance we might attempt to take a screenshot before the first
   // paint, which would fail.
-  Sleep(base::TimeDelta::FromMilliseconds(100));
+  Sleep(base::TimeDelta::FromMilliseconds(200));
 
   // Before navigating away from the red page, we should take a thumbnail.
   // Note that the page load is deliberately slowed down, so that the