Document that the memory increases appear to be necessary for Travis …

…only (and that they do appear to solve the problem there).

RELNOTES=n/a

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=235776445

guava-gwt/pom.xml

@@ -342,6 +342,7 @@
               <mode>htmlunit</mode>
               <htmlunit>FF38</htmlunit>
               <productionMode>true</productionMode>
+              <!-- Fix OutOfMemoryError under Travis. -->
               <extraJvmArgs>-Xms3500m -Xmx3500m -Xss1024k</extraJvmArgs>
               <sourceLevel>1.8</sourceLevel>
               <!-- Keep these timeouts very large because, if we hit the timeout, the tests silently pass :( -->