[turbofan] Bump CallApiCallbackStub::kArgBits to 7.

This allows TurboFan to inline API function calls with more than 7
parameters. It's now possible to inline API function calls with up
to 127 parameters, which should be sufficiently large.

Bug: chromium:794865
Change-Id: I974fed740cd3506d0b8d75ed922227364c51a1fb
Reviewed-on: https://chromium-review.googlesource.com/848712Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50335}

src/code-stubs.h

@@ -582,7 +582,7 @@ class KeyedStoreSloppyArgumentsStub : public TurboFanCodeStub {
 
 class CallApiCallbackStub : public PlatformCodeStub {
  public:
-  static const int kArgBits = 3;
+  static const int kArgBits = 7;
   static const int kArgMax = (1 << kArgBits) - 1;
 
   CallApiCallbackStub(Isolate* isolate, int argc)