Add a SkRWBuffer reserve mechanism

Currently, Chromium stores segmented data in a SharedBuffer and appends
to SkRWBuffer one segment at a time:

        const char* segment = 0;
        for (size_t length = data->getSomeData(segment, m_rwBuffer->size());
            length; length = data->getSomeData(segment, m_rwBuffer->size())) {
            m_rwBuffer->append(segment, length, remaining);
        }

This can yield a bunch of just-above-4k allocations => wasted RAM due to
internal fragmentation.

Ideally, we'd want a SkRWBuffer::reserve(size_t bytes) API, but the
current internals don't support that trivially.

Alternatively, the caller can pass a reserve hint at append() time.

BUG=chromium:651698
R=scroggo@google.com,reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2385803002

Review-Url: https://codereview.chromium.org/2385803002

include/core/SkRWBuffer.h

@@ -79,7 +79,15 @@ public:
     ~SkRWBuffer();
 
     size_t size() const { return fTotalUsed; }
-    void append(const void* buffer, size_t length);
+
+    /**
+     *  Append |length| bytes from |buffer|.
+     *
+     *  If the caller knows in advance how much more data they are going to append, they can
+     *  pass a |reserve| hint (representing the number of upcoming bytes *in addition* to the
+     *  current append), to minimize the number of internal allocations.
+     */
+    void append(const void* buffer, size_t length, size_t reserve = 0);
 
     SkROBuffer* newRBufferSnapshot() const;
     SkStreamAsset* newStreamSnapshot() const;

src/core/SkRWBuffer.cpp

@@ -199,7 +199,7 @@ SkRWBuffer::~SkRWBuffer() {
 // next, since our reader will be using fCapacity (min'd against its total available) to know how
 // many bytes to read from a given block.
 //
-void SkRWBuffer::append(const void* src, size_t length) {
+void SkRWBuffer::append(const void* src, size_t length, size_t reserve) {
     this->validate();
     if (0 == length) {
         return;
@@ -208,7 +208,7 @@ void SkRWBuffer::append(const void* src, size_t length) {
     fTotalUsed += length;
 
     if (nullptr == fHead) {
-        fHead = SkBufferHead::Alloc(length);
+        fHead = SkBufferHead::Alloc(length + reserve);
         fTail = &fHead->fBlock;
     }
 
@@ -218,7 +218,7 @@ void SkRWBuffer::append(const void* src, size_t length) {
     length -= written;
 
     if (length) {
-        SkBufferBlock* block = SkBufferBlock::Alloc(length);
+        SkBufferBlock* block = SkBufferBlock::Alloc(length + reserve);
         fTail->fNext = block;
         fTail = block;
         written = fTail->append(src, length);