Validate that the mip level is not too large for CopyTextureCHROMIUM.

When the mip level was greater than the maximum, it could cause an
out-of-bounds read when trying to query the Texture's size and format
later in the validation.

BUG=740426

Change-Id: Ibd6c977981e5f8b7368c161b7969d9e916c6095b
diff --git a/src/libANGLE/validationES2.cpp b/src/libANGLE/validationES2.cpp
index 5d7d230..e817390 100644
--- a/src/libANGLE/validationES2.cpp
+++ b/src/libANGLE/validationES2.cpp
@@ -342,7 +342,7 @@
 
 bool IsValidCopyTextureSourceLevel(Context *context, GLenum target, GLint level)
 {
-    if (level < 0)
+    if (!ValidMipLevel(context, target, level))
     {
         return false;
     }
@@ -361,7 +361,7 @@
                                         GLsizei width,
                                         GLsizei height)
 {
-    if (level < 0)
+    if (!ValidMipLevel(context, target, level))
     {
         return false;
     }