Fix Layout initialization in the skipCache path

C++ local var initialization always tricks me.  Previously, Layout
didn't have a constructor, which meant that defining it on the stack
left mAdvance uninitialized.  This was not an issue when we were doing
"new Layout()", since that invokes zero-initialization, but was an
issue for the skipCache path that was allocating layout on stack by
just "Layout l" instead of "Layout l = Layout()".  To avoid surprises,
add a constructors that clears everything.

Also adds reset() method to reset the layout for reuse.

Change-Id: I3e02f00da9dd7d360abe13f63c310f6882292d0a

engine/src/flutter/include/minikin/Layout.h

@@ -64,6 +64,14 @@ class LayoutContext;
 // extend through the lifetime of the Layout object.
 class Layout {
 public:
+
+    Layout() : mGlyphs(), mAdvances(), mCollection(0), mFaces(), mAdvance(0), mBounds() {
+        mBounds.setEmpty();
+    }
+
+    // Clears layout, ready to be used again
+    void reset();
+
     void dump() const;
     void setFontCollection(const FontCollection* collection);
 
@@ -72,8 +80,7 @@ public:
 
     void draw(Bitmap*, int x0, int y0, float size) const;
 
-    // This must be called before any invocations.
+    // Deprecated. Nont needed. Remove when callers are removed.
-    // TODO: probably have a factory instead
     static void init();
 
     // public accessors

engine/src/flutter/libs/minikin/Layout.cpp

@@ -267,10 +267,18 @@ void MinikinRect::join(const MinikinRect& r) {
     }
 }
 
-// TODO: the actual initialization is deferred, maybe make this explicit
+// Deprecated. Remove when callers are removed.
 void Layout::init() {
 }
 
+void Layout::reset() {
+    mGlyphs.clear();
+    mFaces.clear();
+    mBounds.setEmpty();
+    mAdvances.clear();
+    mAdvance = 0;
+}
+
 void Layout::setFontCollection(const FontCollection* collection) {
     mCollection = collection;
 }
@@ -519,12 +527,9 @@ void Layout::doLayout(const uint16_t* buf, size_t start, size_t count, size_t bu
     bool isRtl = (bidiFlags & kDirection_Mask) != 0;
     bool doSingleRun = true;
 
-    mGlyphs.clear();
+    reset();
-    mFaces.clear();
-    mBounds.setEmpty();
-    mAdvances.clear();
     mAdvances.resize(count, 0);
-    mAdvance = 0;
+
     if (!(bidiFlags == kBidi_Force_LTR || bidiFlags == kBidi_Force_RTL)) {
         UBiDi* bidi = ubidi_open();
         if (bidi) {