Make SkString movable.

This adds a move constructor and move assignment to SkString. This
allows elision of atomic increments and decrements on the fRec.

TBR=reed
Already agreed that moving is good.

Review URL: https://codereview.chromium.org/1672123002

include/core/SkString.h

@@ -126,6 +126,7 @@ public:
     explicit    SkString(const char text[]);
                 SkString(const char text[], size_t len);
                 SkString(const SkString&);
+                SkString(SkString&&);
                 ~SkString();
 
     bool        isEmpty() const { return 0 == fRec->fLength; }
@@ -172,6 +173,7 @@ public:
     // these methods edit the string
 
     SkString& operator=(const SkString&);
+    SkString& operator=(SkString&&);
     SkString& operator=(const char text[]);
 
     char* writable_str();

src/core/SkString.cpp

@@ -275,6 +275,13 @@ SkString::SkString(const SkString& src) {
     fRec = RefRec(src.fRec);
 }
 
+SkString::SkString(SkString&& src) {
+    src.validate();
+
+    fRec = src.fRec;
+    src.fRec = const_cast<Rec*>(&gEmptyRec);
+}
+
 SkString::~SkString() {
     this->validate();
 
@@ -310,6 +317,15 @@ SkString& SkString::operator=(const SkString& src) {
     return *this;
 }
 
+SkString& SkString::operator=(SkString&& src) {
+    this->validate();
+
+    if (fRec != src.fRec) {
+        this->swap(src);
+    }
+    return *this;
+}
+
 SkString& SkString::operator=(const char text[]) {
     this->validate();