Bug 1524890 - P9. Add += and -= operator. r=gerald

Differential Revision: https://phabricator.services.mozilla.com/D20167

--HG--
extra : moz-landing-system : lando

dom/media/Intervals.h

@@ -88,7 +88,6 @@ class Interval {
     return SelfType(mStart + aVal, mEnd + aVal, mFuzz);
   }
 
-  // Basic interval arithmetic operator definition.
   SelfType operator-(const SelfType& aOther) const {
     return SelfType(mStart - aOther.mEnd, mEnd - aOther.mStart,
                     mFuzz + aOther.mFuzz);
@@ -98,6 +97,32 @@ class Interval {
     return SelfType(mStart - aVal, mEnd - aVal, mFuzz);
   }
 
+  SelfType& operator+=(const SelfType& aOther) {
+    mStart += aOther.mStart;
+    mEnd += aOther.mEnd;
+    mFuzz += aOther.mFuzz;
+    return *this;
+  }
+
+  SelfType& operator+=(const T& aVal) {
+    mStart += aVal;
+    mEnd += aVal;
+    return *this;
+  }
+
+  SelfType& operator-=(const SelfType& aOther) {
+    mStart -= aOther.mStart;
+    mEnd -= aOther.mEnd;
+    mFuzz += aOther.mFuzz;
+    return *this;
+  }
+
+  SelfType& operator-=(const T& aVal) {
+    mStart -= aVal;
+    mEnd -= aVal;
+    return *this;
+  }
+
   bool operator==(const SelfType& aOther) const {
     return mStart == aOther.mStart && mEnd == aOther.mEnd;
   }
@@ -540,7 +565,6 @@ class IntervalSet {
     return false;
   }
 
-
   bool Intersects(const ElemType& aInterval) const {
     for (const auto& interval : mIntervals) {
       if (interval.Intersects(aInterval)) {