Extract hardcoded error strings into a single place and replace them with enum.

src/lithium.cc

Index: src/lithium.cc
diff --git a/src/lithium.cc b/src/lithium.cc
index e9c3531e385af6834b888c9ec2a230ab1e2b3569..790a2182b176ac5d86e701e01395297d90c6e5ca 100644
--- a/src/lithium.cc
+++ b/src/lithium.cc
@@ -425,7 +425,7 @@ LChunk* LChunk::NewChunk(HGraph* graph) {
   int values = graph->GetMaximumValueID();
   CompilationInfo* info = graph->info();
   if (values > LUnallocated::kMaxVirtualRegisters) {
-    info->set_bailout_reason("not enough virtual registers for values");
+    info->set_bailout_reason(kNotEnoughVirtualRegistersForValues);
     return NULL;
   }
   LAllocator allocator(values, graph);
@@ -434,7 +434,7 @@ LChunk* LChunk::NewChunk(HGraph* graph) {
   if (chunk == NULL) return NULL;
 
   if (!allocator.Allocate(chunk)) {
-    info->set_bailout_reason("not enough virtual registers (regalloc)");
+    info->set_bailout_reason(kNotEnoughVirtualRegistersRegalloc);
     return NULL;
   }