Add overrides and switch to nullptr keyword

Changes generated with clang-modernize.

Additionally, fixed some struct-vs-class usage to make clang happy.

Change-Id: Ic6ef2427401ff1e794d26f21f7b44868fc75fb72
diff --git a/libs/hwui/TextDropShadowCache.h b/libs/hwui/TextDropShadowCache.h
index bb53a23..caf089f 100644
--- a/libs/hwui/TextDropShadowCache.h
+++ b/libs/hwui/TextDropShadowCache.h
@@ -34,8 +34,8 @@
 class FontRenderer;
 
 struct ShadowText {
-    ShadowText(): len(0), radius(0.0f), textSize(0.0f), typeface(NULL),
-            flags(0), italicStyle(0.0f), scaleX(0), text(NULL), positions(NULL) {
+    ShadowText(): len(0), radius(0.0f), textSize(0.0f), typeface(nullptr),
+            flags(0), italicStyle(0.0f), scaleX(0), text(nullptr), positions(nullptr) {
     }
 
     // len is the number of bytes in text
@@ -76,7 +76,7 @@
         uint32_t charCount = len / sizeof(char16_t);
         str.setTo((const char16_t*) text, charCount);
         text = str.string();
-        if (positions != NULL) {
+        if (positions != nullptr) {
             positionsCopy.clear();
             positionsCopy.appendArray(positions, charCount * 2);
             positions = positionsCopy.array();
@@ -134,7 +134,7 @@
      * Used as a callback when an entry is removed from the cache.
      * Do not invoke directly.
      */
-    void operator()(ShadowText& text, ShadowTexture*& texture);
+    void operator()(ShadowText& text, ShadowTexture*& texture) override;
 
     ShadowTexture* get(const SkPaint* paint, const char* text, uint32_t len,
             int numGlyphs, float radius, const float* positions);