Make grokdump.py work on Windows

mmap behaves differently on Windows. This change adjust how grokdump.py
uses it so that the script will run on Windows.

The disassembly doesn't work due to lack of /usr/bin/objdump - fixing
that is out of scope for this change. The output is still useful even
without the disassembly.

Change-Id: I0db2d09e3ed10f0ca666cbbd438fbd82565906d8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1866958
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64376}

tools/grokdump.py

@@ -616,8 +616,12 @@ class MinidumpReader(object):
 
   def __init__(self, options, minidump_name):
     self.minidump_name = minidump_name
-    self.minidump_file = open(minidump_name, "r")
-    self.minidump = mmap.mmap(self.minidump_file.fileno(), 0, mmap.MAP_PRIVATE)
+    if sys.platform == 'win32':
+      self.minidump_file = open(minidump_name, "a+")
+      self.minidump = mmap.mmap(self.minidump_file.fileno(), 0)
+    else:
+      self.minidump_file = open(minidump_name, "r")
+      self.minidump = mmap.mmap(self.minidump_file.fileno(), 0, mmap.MAP_PRIVATE)
     self.header = MINIDUMP_HEADER.Read(self.minidump, 0)
     if self.header.signature != MinidumpReader._HEADER_MAGIC:
       print("Warning: Unsupported minidump header magic!", file=sys.stderr)