Avoid boxing ints in ConcurrentHashMultiset Preconditions checks

The preconditions checks for occurrences are boxing int values for the format
string. This has shown up in profiling as we are using this collection quite
heavily. Switch from preconditions formatting to basic if statement to avoid boxing.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=101702798

guava/src/com/google/common/collect/CollectPreconditions.java

@@ -41,6 +41,12 @@ static int checkNonnegative(int value, String name) {
     return value;
   }
 
+  static void checkPositive(int value, String name) {
+    if (value <= 0) {
+      throw new IllegalArgumentException(name + " must be positive but was: " + value);
+    }
+  }
+
   /**
    * Precondition tester for {@code Iterator.remove()} that throws an exception with a consistent
    * error message.

guava/src/com/google/common/collect/ConcurrentHashMultiset.java

@@ -220,7 +220,7 @@ public int add(E element, int occurrences) {
     if (occurrences == 0) {
       return count(element);
     }
-    checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);
+    CollectPreconditions.checkPositive(occurrences, "occurences");
 
     while (true) {
       AtomicInteger existingCounter = Maps.safeGet(countMap, element);
@@ -285,7 +285,7 @@ public int remove(@Nullable Object element, int occurrences) {
     if (occurrences == 0) {
       return count(element);
     }
-    checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);
+    CollectPreconditions.checkPositive(occurrences, "occurences");
 
     AtomicInteger existingCounter = Maps.safeGet(countMap, element);
     if (existingCounter == null) {
@@ -325,7 +325,7 @@ public boolean removeExactly(@Nullable Object element, int occurrences) {
     if (occurrences == 0) {
       return true;
     }
-    checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);
+    CollectPreconditions.checkPositive(occurrences, "occurences");
 
     AtomicInteger existingCounter = Maps.safeGet(countMap, element);
     if (existingCounter == null) {