Give Compiler a back reference to the driver.

The compiler driver is a single object delegating work to the compiler, rather
than passing it through to every Compiler call make it a member of Compiler so
that it maybe queried. This simplifies the Compiler API and makes the
relationship to CompilerDriver more explicit.
Remove reference arguments that contravene code style.

Change-Id: Iba47f2e3cbda679a7ec7588f26188d77643aa2c6
diff --git a/compiler/dex/frontend.cc b/compiler/dex/frontend.cc
index 1a9379a..5e13722 100644
--- a/compiler/dex/frontend.cc
+++ b/compiler/dex/frontend.cc
@@ -30,12 +30,12 @@
 
 namespace art {
 
-extern "C" void ArtInitQuickCompilerContext(art::CompilerDriver& driver) {
-  CHECK(driver.GetCompilerContext() == NULL);
+extern "C" void ArtInitQuickCompilerContext(art::CompilerDriver* driver) {
+  CHECK(driver->GetCompilerContext() == nullptr);
 }
 
-extern "C" void ArtUnInitQuickCompilerContext(art::CompilerDriver& driver) {
-  CHECK(driver.GetCompilerContext() == NULL);
+extern "C" void ArtUnInitQuickCompilerContext(art::CompilerDriver* driver) {
+  CHECK(driver->GetCompilerContext() == nullptr);
 }
 
 /* Default optimizer/debug setting for the compiler. */