Revert "Disable flaky PlatformUtil tests."

This reverts commit 589d0717.

Reason for revert: We believe this is fixed on crbug.com/1004639. We'll monitor the bots.

Original change's description:
> Disable flaky PlatformUtil tests.
> 
> When these two tests are run together, one always fails. MSAN reveals
> that there is some use-after-free in these tests. Disable them for now.
> 
> Recent flakiness: https://ci.chromium.org/p/chromium/builders/try/linux-chromeos-rel/384207
> Flakiness dashboard: https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=unit_tests&tests=PlatformUtilPosixTest.
> 
> TBR=avi@chromium.org
> BUG=1007240
> 
> Change-Id: I961d3ed160821bd82299944cba2e827887fad7bd
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1821700
> Reviewed-by: Dominick Ng <dominickn@chromium.org>
> Commit-Queue: Dominick Ng <dominickn@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#699316}

TBR=avi@chromium.org,dominickn@chromium.org

Change-Id: I4c11f5b55fbded03a1d2f1af4f3e0a4ed3fab590
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1007240
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824518Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699587}

chrome/browser/platform_util_unittest.cc

@@ -246,8 +246,7 @@ class PlatformUtilPosixTest : public PlatformUtilTest {
 // ChromeOS doesn't follow symbolic links in sandboxed filesystems. So all the
 // symbolic link tests should return PATH_NOT_FOUND.
 
-// Very flaky due to memory unsafety: crbug.com/1007240
-TEST_F(PlatformUtilPosixTest, DISABLED_OpenFileWithPosixSymlinksChromeOS) {
+TEST_F(PlatformUtilPosixTest, OpenFileWithPosixSymlinksChromeOS) {
   EXPECT_EQ(OPEN_FAILED_PATH_NOT_FOUND,
             CallOpenItem(symlink_to_file_, OPEN_FILE));
   EXPECT_EQ(OPEN_FAILED_PATH_NOT_FOUND,
@@ -265,8 +264,7 @@ TEST_F(PlatformUtilPosixTest, OpenFolderWithPosixSymlinksChromeOS) {
             CallOpenItem(symlink_to_nowhere_, OPEN_FOLDER));
 }
 
-// Very flaky due to memory unsafety: crbug.com/1007240
-TEST_F(PlatformUtilTest, DISABLED_OpenFileWithUnhandledFileType) {
+TEST_F(PlatformUtilTest, OpenFileWithUnhandledFileType) {
   base::FilePath unhandled_file =
       directory_.GetPath().AppendASCII("myfile.filetype");
   ASSERT_EQ(3, base::WriteFile(unhandled_file, "cat", 3));