Reduce memory peaks when generating data urls for images.

Source/core/inspector/InspectorLayerTreeAgent.cpp

Index: Source/core/inspector/InspectorLayerTreeAgent.cpp
diff --git a/Source/core/inspector/InspectorLayerTreeAgent.cpp b/Source/core/inspector/InspectorLayerTreeAgent.cpp
index 27eb0d398047cde607bcd99e777f691e694f2dc3..88ae87c6f415a9cfb3a7db965ac4e3612ce61c7c 100644
--- a/Source/core/inspector/InspectorLayerTreeAgent.cpp
+++ b/Source/core/inspector/InspectorLayerTreeAgent.cpp
@@ -53,6 +53,7 @@
 #include "public/platform/WebFloatPoint.h"
 #include "public/platform/WebLayer.h"
 #include "wtf/text/Base64.h"
+#include "wtf/text/StringBuilder.h"
 
 namespace blink {
 
@@ -383,7 +384,11 @@ void InspectorLayerTreeAgent::replaySnapshot(ErrorString* errorString, const Str
         *errorString = "Image encoding failed";
         return;
     }
-    *dataURL = "data:image/png;base64," + *base64Data;
+    StringBuilder url;

[pdr., 2014/09/11 04:38:15]

Any reason for not using reserveCapacity before appending the string literal?

You may have seen this already but the new string doc is a good read:
https://docs.google.com/a/google.com/document/d/1kOCUlJdh2WJMJGDf-WoEQhmnjKLa...

[Daniel Bratell, 2014/09/11 11:22:31]

I don't think I had seen that document before. It is a high level document
though and I'm digging around in the dark sides of those objects. For example,
it says that operator+ is good, but it's also O(n^2) in the number of pluses so
it too can become a bad choice.

The reason I didn't do reserveCapacity(22 + base64Data->size()) or
reserveCapacity(sizeof("data:image/png;base64,") - 1 + base64Data->size()) or
reserveCapacity(strlen("data:image/png;base64,") + base64Data->size()) was to
make the code short and easy to read. This whole block will be totally dominated
by the append of the image data so there is nothing to gain from optimizing it
further.

+    url.append("data:image/png;base64,");

[Noel Gordon, 2014/09/11 02:39:45]

per https://trac.webkit.org/wiki/EfficientStrings, I think want .appendLiteral
here.

+    url.reserveCapacity(url.length() + base64Data->size()); // Prevent over-allocation.

[Noel Gordon, 2014/09/11 01:03:23]

nit: remove the comment.

[Daniel Bratell, 2014/09/11 11:22:31]

Done.

+    url.append(base64Data->begin(), base64Data->size());
+    *dataURL = url.toString();
 }
 
 void InspectorLayerTreeAgent::profileSnapshot(ErrorString* errorString, const String& snapshotId, const int* minRepeatCount, const double* minDuration, RefPtr<TypeBuilder::Array<TypeBuilder::Array<double> > >& outTimings)