Re-enabling previously-flaky CrossSiteDocument*.BlockForVariousTargets

This CL re-enables CrossSiteDocumentBlockingTest.BlockForVariousTargets
because we speculate (as explained in https://crbug.com/1075663#c15)
that the root cause of the flakiness was unrelated to the test and has
been fixed by r792326 which fixed raciness in NestedMessagePumpAndroid
in July.

Fixed: 1075663
Change-Id: Ie370c65671d9d498056ffb95c4e75e8940e8d1a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508910Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822689}

content/browser/loader/cross_site_document_blocking_browsertest.cc

@@ -770,14 +770,7 @@ IN_PROC_BROWSER_TEST_P(CrossSiteDocumentBlockingTest, BackToAboutBlank) {
   }
 }
 
-#if defined(OS_ANDROID)
-// Test is flaky on Android, see crbug.com/1075663
-#define MAYBE_BlockForVariousTargets DISABLED_BlockForVariousTargets
-#else
-#define MAYBE_BlockForVariousTargets BlockForVariousTargets
-#endif
-IN_PROC_BROWSER_TEST_P(CrossSiteDocumentBlockingTest,
-                       MAYBE_BlockForVariousTargets) {
+IN_PROC_BROWSER_TEST_P(CrossSiteDocumentBlockingTest, BlockForVariousTargets) {
   // This webpage loads a cross-site HTML page in different targets such as
   // <img>,<link>,<embed>, etc. Since the requested document is blocked, and one
   // character string (' ') is returned instead, this tests that the renderer