Bug 1342720 - Optimize away the cost of creating and destroying nsBidi from CanvasRenderingContext2D::DrawOrMeasureText(); r=baku

The ICU implementation of ~nsBidi is particularly expensive because
it calls free() several times.

dom/canvas/CanvasRenderingContext2D.cpp

@@ -57,7 +57,6 @@
 #include "gfxUtils.h"
 
 #include "nsFrameLoader.h"
-#include "nsBidi.h"
 #include "nsBidiPresUtils.h"
 #include "Layers.h"
 #include "LayerUserData.h"
@@ -4276,7 +4275,6 @@ CanvasRenderingContext2D::DrawOrMeasureText(const nsAString& aRawText,
 
   // calls bidi algo twice since it needs the full text width and the
   // bounding boxes before rendering anything
-  nsBidi bidiEngine;
   rv = nsBidiPresUtils::ProcessText(textToDraw.get(),
                                     textToDraw.Length(),
                                     isRTL ? NSBIDI_RTL : NSBIDI_LTR,
@@ -4286,7 +4284,7 @@ CanvasRenderingContext2D::DrawOrMeasureText(const nsAString& aRawText,
                                     nullptr,
                                     0,
                                     &totalWidthCoord,
-                                    &bidiEngine);
+                                    &mBidiEngine);
   if (NS_FAILED(rv)) {
     return rv;
   }
@@ -4401,7 +4399,7 @@ CanvasRenderingContext2D::DrawOrMeasureText(const nsAString& aRawText,
                                     nullptr,
                                     0,
                                     nullptr,
-                                    &bidiEngine);
+                                    &mBidiEngine);
 
 
   mTarget->SetTransform(oldTransform);

dom/canvas/CanvasRenderingContext2D.h

@@ -29,6 +29,7 @@
 #include "FilterSupport.h"
 #include "nsSVGEffects.h"
 #include "Layers.h"
+#include "nsBidi.h"
 
 class nsGlobalWindow;
 class nsXULElement;
@@ -864,6 +865,8 @@ protected:
 
   nsTArray<RegionInfo> mHitRegionsOptions;
 
+  nsBidi mBidiEngine;
+
   /**
     * Returns true if a shadow should be drawn along with a
     * drawing operation.