Attempt to deflake SplitPreloaderTest

I wasn't able to repro the flake locally, but this is my best guess at a
fix.

Bug: 1166286
Change-Id: I2d229d66ea0f16f2c2f959642cdc5baf1598fdef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628016
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Auto-Submit: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843350}

chrome/android/junit/src/org/chromium/chrome/browser/base/SplitPreloaderTest.java

@@ -71,7 +71,9 @@ public class SplitPreloaderTest {
             if (ThreadUtils.runningOnUiThread()) {
                 mUiThreadContextNames.add(name);
             } else {
-                mBackgroundThreadContextNames.add(name);
+                synchronized (mBackgroundThreadContextNames) {
+                    mBackgroundThreadContextNames.add(name);
+                }
             }
             return new SplitContext(this, name);
         }
@@ -81,7 +83,9 @@ public class SplitPreloaderTest {
         }
 
         public List<String> getBackgroundThreadContextNames() {
-            return mBackgroundThreadContextNames;
+            synchronized (mBackgroundThreadContextNames) {
+                return new ArrayList<>(mBackgroundThreadContextNames);
+            }
         }
     }