BufferQueue consumers: Add discardFreeBuffer method

This method releases all free buffers owned by the buffer queue,
in order to save memory (at the cost of potential future
reallocation of buffers).

Bug: 28695173
Change-Id: I458d10373e639e3144faf673af2ba01aca36e65a
diff --git a/libs/gui/ConsumerBase.cpp b/libs/gui/ConsumerBase.cpp
index 84965ef..a1bdf4a 100644
--- a/libs/gui/ConsumerBase.cpp
+++ b/libs/gui/ConsumerBase.cpp
@@ -245,6 +245,15 @@
     return mConsumer->getOccupancyHistory(forceFlush, outHistory);
 }
 
+status_t ConsumerBase::discardFreeBuffers() {
+    Mutex::Autolock _l(mMutex);
+    if (mAbandoned) {
+        CB_LOGE("discardFreeBuffers: ConsumerBase is abandoned!");
+        return NO_INIT;
+    }
+    return mConsumer->discardFreeBuffers();
+}
+
 void ConsumerBase::dump(String8& result) const {
     dump(result, "");
 }