Refactor in preparation of adding OES_depth_texture

Rename existing depthTexture extension flag to depthTextureANGLE to
distinguish it from OES depth texture extension to be added.

Bug: angleproject:3103
Test: angle_end2end_tests --gtest_filter=DepthStencilFormatsTest.DepthTexture
Change-Id: I44c69a69b925a84f931518e0374e662893813061
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1575425
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/validationES2.cpp b/src/libANGLE/validationES2.cpp
index 254db5b..c4e05e1 100644
--- a/src/libANGLE/validationES2.cpp
+++ b/src/libANGLE/validationES2.cpp
@@ -732,7 +732,7 @@
             case GL_DEPTH_COMPONENT32_OES:
             case GL_DEPTH_STENCIL_OES:
             case GL_DEPTH24_STENCIL8_OES:
-                if (context->getExtensions().depthTextures)
+                if (context->getExtensions().depthTextureANGLE)
                 {
                     context->validationError(GL_INVALID_OPERATION, kInvalidFormat);
                     return false;
@@ -1497,7 +1497,7 @@
                 break;
             case GL_DEPTH_COMPONENT:
             case GL_DEPTH_STENCIL_OES:
-                if (!context->getExtensions().depthTextures)
+                if (!context->getExtensions().depthTextureANGLE)
                 {
                     context->validationError(GL_INVALID_ENUM, kEnumNotSupported);
                     return false;
@@ -1588,7 +1588,7 @@
 
                 case GL_DEPTH_COMPONENT:
                 case GL_DEPTH_STENCIL:
-                    if (!context->getExtensions().depthTextures)
+                    if (!context->getExtensions().depthTextureANGLE)
                     {
                         context->validationError(GL_INVALID_ENUM, kInvalidFormat);
                         return false;
@@ -1884,7 +1884,7 @@
         case GL_DEPTH_COMPONENT16:
         case GL_DEPTH_COMPONENT32_OES:
         case GL_DEPTH24_STENCIL8_OES:
-            if (!context->getExtensions().depthTextures)
+            if (!context->getExtensions().depthTextureANGLE)
             {
                 context->validationError(GL_INVALID_ENUM, kEnumNotSupported);
                 return false;