Clean up TSymbolTable entry points

TSymbolTable API can be cleaned up further now that we have separate
logic for inserting builtins and user-defined symbols.

BUG=angleprojec:2267
TEST=angle_unittests

Change-Id: I7a228891ecdf4696e50868b9e7dfc2a50b5d8e92
Reviewed-on: https://chromium-review.googlesource.com/941301
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/compiler/translator/Compiler.cpp b/src/compiler/translator/Compiler.cpp
index 7c685d5..b6a41cc 100644
--- a/src/compiler/translator/Compiler.cpp
+++ b/src/compiler/translator/Compiler.cpp
@@ -193,12 +193,12 @@
   public:
     TScopedSymbolTableLevel(TSymbolTable *table) : mTable(table)
     {
-        ASSERT(mTable->atBuiltInLevel());
+        ASSERT(mTable->isEmpty());
         mTable->push();
     }
     ~TScopedSymbolTableLevel()
     {
-        while (!mTable->atBuiltInLevel())
+        while (!mTable->isEmpty())
             mTable->pop();
     }