Bug 968520 - Use FallibleTArray::Clear instead of SetLength(0). r=froydnj

This preemptively fixes the upcoming -Wunused-result warnings due to unchecked
SetLength calls.

dom/canvas/WebGLElementArrayCache.cpp

@@ -377,7 +377,7 @@ WebGLElementArrayCacheTree<T>::Update(size_t firstByte, size_t lastByte)
     if (requiredNumLeaves != NumLeaves()) {
         // See class comment for why we the tree storage size is 2 * numLeaves.
         if (!mTreeData.SetLength(2 * requiredNumLeaves, fallible)) {
-            mTreeData.SetLength(0);
+            mTreeData.Clear();
             return false;
         }
         MOZ_ASSERT(NumLeaves() == requiredNumLeaves);
@@ -471,7 +471,7 @@ WebGLElementArrayCache::BufferData(const void* ptr, size_t byteLength)
 {
     if (mBytes.Length() != byteLength) {
         if (!mBytes.SetLength(byteLength, fallible)) {
-            mBytes.SetLength(0);
+            mBytes.Clear();
             return false;
         }
     }

dom/crypto/CryptoBuffer.cpp

@@ -59,7 +59,7 @@ CryptoBuffer::Assign(const ArrayBufferViewOrArrayBuffer& aData)
 
   // If your union is uninitialized, something's wrong
   MOZ_ASSERT(false);
-  SetLength(0);
+  Clear();
   return nullptr;
 }
 
@@ -74,7 +74,7 @@ CryptoBuffer::Assign(const OwningArrayBufferViewOrArrayBuffer& aData)
 
   // If your union is uninitialized, something's wrong
   MOZ_ASSERT(false);
-  SetLength(0);
+  Clear();
   return nullptr;
 }