Revert "Revert "Stop using the runtime classpath in dex2oat""

This reverts commit 053a84b762536144d5cae309ca5abd97766b4583.

Reason for revert: Prepare the CL again to be submitted after
https://chrome-internal-review.googlesource.com/c/423809 gets merged.

Change-Id: I1d0e3d6bf2aa16249b2d0146acd9c81149924c00
diff --git a/runtime/class_loader_context.h b/runtime/class_loader_context.h
index 85299d4..9afa880 100644
--- a/runtime/class_loader_context.h
+++ b/runtime/class_loader_context.h
@@ -34,9 +34,6 @@
 // Utility class which holds the class loader context used during compilation/verification.
 class ClassLoaderContext {
  public:
-  // Creates an empty context (with no class loaders).
-  ClassLoaderContext();
-
   ~ClassLoaderContext();
 
   // Opens requested class path files and appends them to ClassLoaderInfo::opened_dex_files.
@@ -126,6 +123,10 @@
   static std::unique_ptr<ClassLoaderContext> CreateContextForClassLoader(jobject class_loader,
                                                                          jobjectArray dex_elements);
 
+  // Returns the default class loader context to be used when none is specified.
+  // This will return a context with a single and empty PathClassLoader.
+  static std::unique_ptr<ClassLoaderContext> Default();
+
  private:
   enum ClassLoaderType {
     kInvalidClassLoader = 0,
@@ -151,6 +152,9 @@
     explicit ClassLoaderInfo(ClassLoaderType cl_type) : type(cl_type) {}
   };
 
+  // Creates an empty context (with no class loaders).
+  ClassLoaderContext();
+
   // Constructs an empty context.
   // `owns_the_dex_files` specifies whether or not the context will own the opened dex files
   // present in the class loader chain. If `owns_the_dex_files` is true then OpenDexFiles cannot