Make unique id counter a member of TSymbolTable

This makes unique id counting thread-safe.

BUG=angleproject:624
TEST=angle_unittests

Change-Id: Ie0f2c7e574470b39750d37d2181c790bc874b275
Reviewed-on: https://chromium-review.googlesource.com/570419
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/compiler/translator/EmulatePrecision.cpp b/src/compiler/translator/EmulatePrecision.cpp
index 9f56766..ba09fd7 100644
--- a/src/compiler/translator/EmulatePrecision.cpp
+++ b/src/compiler/translator/EmulatePrecision.cpp
@@ -513,7 +513,7 @@
 
 }  // namespace anonymous
 
-EmulatePrecision::EmulatePrecision(const TSymbolTable &symbolTable, int shaderVersion)
+EmulatePrecision::EmulatePrecision(TSymbolTable *symbolTable, int shaderVersion)
     : TLValueTrackingTraverser(true, true, true, symbolTable, shaderVersion),
       mDeclaringVariables(false)
 {