Refactor LinuxDumper and MinidumpWriter.

This patch is part of a bigger patch that helps merging the breakpad code
with the modified version in Chromium OS.

Specifically, this patch makes the following changes:
1. Add two convenient methods, back() and empty(), to the wasteful_vector
   class.
2. Refactor the LinuxDumper class such that it can later be splitted into
   a base class and two derived classes, one uses the current ptrace
   implementation and one uses a core file.
3. Refactor the MinidumpWriter class such that it can later use different
   derived implementations of LinuxDumper.

BUG=455
TEST=Tested the following:
1. Build on 32-bit and 64-bit Linux with gcc 4.4.3 and gcc 4.6.
2. Build on Mac OS X 10.6.8 with gcc 4.2 and clang 3.0 (with latest gmock).
3. All unit tests pass.
Review URL: http://breakpad.appspot.com/340001

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@902 4c0a9323-5329-0410-9bdc-e9ce6186880e

src/common/memory.h

@@ -145,6 +145,18 @@ class wasteful_vector {
         used_(0) {
   }
 
+  T& back() {
+    return a_[used_ - 1];
+  }
+
+  const T& back() const {
+    return a_[used_ - 1];
+  }
+
+  bool empty() const {
+    return used_ == 0;
+  }
+
   void push_back(const T& new_element) {
     if (used_ == allocated_)
       Realloc(allocated_ * 2);