Supply separate flags for onBuildTileIndex

This is a follow up to:
https://codereview.chromium.org/1401283003/

Supply separate flags for onBuildTileIndex

Since png and jpeg's implementations of onBuildTileIndex rely on
modifications to their underlying libraries, rather than whether we are
running on Android, use separate flags that can be disabled
independently.

This will allow us to easily turn off the feature. It also is a step
towards building and running on other platforms for testing (e.g.
valgrind/ASAN to find memory leaks etc).

BUG=skia:

Review URL: https://codereview.chromium.org/1402783008
diff --git a/src/images/SkJpegUtility.cpp b/src/images/SkJpegUtility.cpp
index 10249ea..f1a32ca 100644
--- a/src/images/SkJpegUtility.cpp
+++ b/src/images/SkJpegUtility.cpp
@@ -13,7 +13,7 @@
     skjpeg_source_mgr*  src = (skjpeg_source_mgr*)cinfo->src;
     src->next_input_byte = (const JOCTET*)src->fBuffer;
     src->bytes_in_buffer = 0;
-#ifdef SK_BUILD_FOR_ANDROID
+#ifdef SK_JPEG_INDEX_SUPPORTED
     src->current_offset = 0;
 #endif
     if (!src->fStream->rewind()) {
@@ -22,7 +22,7 @@
     }
 }
 
-#ifdef SK_BUILD_FOR_ANDROID
+#ifdef SK_JPEG_INDEX_SUPPORTED
 static boolean sk_seek_input_data(j_decompress_ptr cinfo, long byte_offset) {
     skjpeg_source_mgr* src = (skjpeg_source_mgr*)cinfo->src;
     size_t bo = (size_t) byte_offset;
@@ -57,7 +57,7 @@
         return FALSE;
     }
 
-#ifdef SK_BUILD_FOR_ANDROID
+#ifdef SK_JPEG_INDEX_SUPPORTED
     src->current_offset += bytes;
 #endif
     src->next_input_byte = (const JOCTET*)src->fBuffer;
@@ -77,7 +77,7 @@
                 cinfo->err->error_exit((j_common_ptr)cinfo);
                 return;
             }
-#ifdef SK_BUILD_FOR_ANDROID
+#ifdef SK_JPEG_INDEX_SUPPORTED
             src->current_offset += bytes;
 #endif
             bytesToSkip -= bytes;
@@ -104,7 +104,7 @@
     skip_input_data = sk_skip_input_data;
     resync_to_restart = jpeg_resync_to_restart;
     term_source = sk_term_source;
-#ifdef SK_BUILD_FOR_ANDROID
+#ifdef SK_JPEG_INDEX_SUPPORTED
     seek_input_data = sk_seek_input_data;
 #endif
 //    SkDebugf("**************** use memorybase %p %d\n", fMemoryBase, fMemoryBaseSize);