Fix float and half float support on mobile.

src/gpu/gl/GrGpuGL.cpp

Index: src/gpu/gl/GrGpuGL.cpp
diff --git a/src/gpu/gl/GrGpuGL.cpp b/src/gpu/gl/GrGpuGL.cpp
index c899eba311f2b8485aaf3a290c0948534b35b1af..0de41318e865193c2668c63b142049c5f374c14e 100644
--- a/src/gpu/gl/GrGpuGL.cpp
+++ b/src/gpu/gl/GrGpuGL.cpp
@@ -2242,16 +2242,18 @@ bool GrGpuGL::configToGLFormats(GrPixelConfig config,
             break;
 
         case kAlpha_half_GrPixelConfig:
-            if (this->glCaps().textureRedSupport()) {
+            if (kGLES_GrGLStandard == this->glStandard() && this->glVersion() < GR_GL_VER(3, 1)) {
+                *internalFormat = GR_GL_ALPHA;
+                *externalFormat = GR_GL_ALPHA;
+                *externalType = GR_GL_HALF_FLOAT_OES;
+            } else if (this->glCaps().textureRedSupport()) {
                 *internalFormat = GR_GL_R16F;
                 *externalFormat = GR_GL_RED;
-                *externalType = (kGLES_GrGLStandard == this->glStandard()) ? GR_GL_HALF_FLOAT
-                                                                           : GR_GL_HALF_FLOAT_OES;
+                *externalType = GR_GL_HALF_FLOAT;
             } else {
                 *internalFormat = GR_GL_ALPHA16F;
                 *externalFormat = GR_GL_ALPHA;
-                *externalType = (kGLES_GrGLStandard == this->glStandard()) ? GR_GL_HALF_FLOAT
-                                                                           : GR_GL_HALF_FLOAT_OES;
+                *externalType = GR_GL_HALF_FLOAT;
             }
             break;