[gdb] Select frame above the DCHECK function

Rather than having GDB always stop on the line containing
V8_IMMEDIATE_CRASH(), walk up the stack looking for V8_Dcheck and select
the frame above it. This will be the frame containing DCHECK (including
related macros like DCHECK_EQ).

Change-Id: I9760e7a4dd78b567dfa77ff12569d287d80ca873
Reviewed-on: https://chromium-review.googlesource.com/1172780Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55112}

tools/gdbinit

@@ -122,3 +122,22 @@ end
 
 set disassembly-flavor intel
 set disable-randomization off
+
+# Install a handler whenever the debugger stops due to a signal. It walks up the
+# stack looking for V8_Dcheck and moves the frame to the one above it so it's
+# immediately at the line of code that triggered the DCHECK.
+python
+def dcheck_stop_handler(event):
+  orig_frame = gdb.selected_frame()
+  frame = orig_frame
+  while frame is not None:
+    if frame.name() == 'V8_Dcheck':
+      select_frame = frame.older()
+      if select_frame is not None:
+        select_frame.select()
+        gdb.execute('frame')
+        break
+    frame = frame.older()
+
+gdb.events.stop.connect(dcheck_stop_handler)
+end