Add caps and GL API for buffer texture

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1784063003

Review URL: https://codereview.chromium.org/1784063003
diff --git a/src/gpu/gl/GrGLDefines.h b/src/gpu/gl/GrGLDefines.h
index 2553002..cd73be4 100644
--- a/src/gpu/gl/GrGLDefines.h
+++ b/src/gpu/gl/GrGLDefines.h
@@ -109,6 +109,7 @@
 #define GR_GL_ARRAY_BUFFER                   0x8892
 #define GR_GL_ELEMENT_ARRAY_BUFFER           0x8893
 #define GR_GL_DRAW_INDIRECT_BUFFER           0x8F3F
+#define GR_GL_TEXTURE_BUFFER                 0x8C2A
 #define GR_GL_ARRAY_BUFFER_BINDING           0x8894
 #define GR_GL_ELEMENT_ARRAY_BUFFER_BINDING   0x8895
 #define GR_GL_DRAW_INDIRECT_BUFFER_BINDING   0x8F43
@@ -236,6 +237,7 @@
 #define GR_GL_RENDERBUFFER_COLOR_SAMPLES     0x8E10
 #define GR_GL_MAX_MULTISAMPLE_COVERAGE_MODES 0x8E11
 #define GR_GL_MULTISAMPLE_COVERAGE_MODES     0x8E12
+#define GR_GL_MAX_TEXTURE_BUFFER_SIZE        0x8C2B
 
 /* GetTextureParameter */
 /*      GL_TEXTURE_MAG_FILTER */