Convert NULL and 0 to nullptr.

This was created by looking at warnings produced by clang's
-Wzero-as-null-pointer-constant. This updates most issues in
Skia code. However, there are places where GL and Vulkan want
pointer values which are explicitly 0, external headers which
use NULL directly, and possibly more uses in un-compiled
sources (for other platforms).

Change-Id: Id22fbac04d5c53497a53d734f0896b4f06fe8345
Reviewed-on: https://skia-review.googlesource.com/39521
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
diff --git a/include/core/SkPaint.h b/include/core/SkPaint.h
index 56daaf3..e2937c9 100644
--- a/include/core/SkPaint.h
+++ b/include/core/SkPaint.h
@@ -739,7 +739,7 @@
         @return     true if the path represents style fill, or false if it represents hairline
     */
     bool getFillPath(const SkPath& src, SkPath* dst) const {
-        return this->getFillPath(src, dst, NULL, 1);
+        return this->getFillPath(src, dst, nullptr, 1);
     }
 
     /** Optional colors used when filling a path, such as a gradient.
@@ -1293,7 +1293,7 @@
 
         @return  recommended spacing between lines
     */
-    SkScalar getFontSpacing() const { return this->getFontMetrics(NULL, 0); }
+    SkScalar getFontSpacing() const { return this->getFontMetrics(nullptr, 0); }
 
     /** Converts text into glyph indices.
         Returns the number of glyph indices represented by text.
@@ -1355,7 +1355,7 @@
         @return            number of glyphs represented by text of length byteLength
     */
     int countText(const void* text, size_t byteLength) const {
-        return this->textToGlyphs(text, byteLength, NULL);
+        return this->textToGlyphs(text, byteLength, nullptr);
     }
 
     /** Returns the advance width of text if kVerticalText_Flag is clear,
@@ -1386,7 +1386,7 @@
         @return        advance width or height
     */
     SkScalar measureText(const void* text, size_t length) const {
-        return this->measureText(text, length, NULL);
+        return this->measureText(text, length, nullptr);
     }
 
     /** Returns the bytes of text that fit within maxWidth.
@@ -1407,7 +1407,7 @@
         @return               bytes of text that fit, always less than or equal to length
     */
     size_t  breakText(const void* text, size_t length, SkScalar maxWidth,
-                      SkScalar* measuredWidth = NULL) const;
+                      SkScalar* measuredWidth = nullptr) const;
 
     /** Retrieves the advance and bounds for each glyph in text, and returns
         the glyph count in text.
@@ -1428,7 +1428,7 @@
         @return            glyph count in text
     */
     int getTextWidths(const void* text, size_t byteLength, SkScalar widths[],
-                      SkRect bounds[] = NULL) const;
+                      SkRect bounds[] = nullptr) const;
 
     /** Returns the geometry as SkPath equivalent to the drawn text.
         Uses SkPaint::TextEncoding to decode text, SkTypeface to get the glyph paths,