Rename [CustomGetter/CustomSetter] to [Custom=Getter|Setter]

Consolidates 3 extended attributes into 1:
[Custom]       => [Custom]
[CustomGetter] => [Custom=Getter]
[CustomSetter] => [Custom=Setter]

No changes to generated code.

Docs updated:
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-Custom-m-a-

Review URL: https://codereview.chromium.org/66033004

git-svn-id: svn://svn.chromium.org/blink/trunk@161717 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move logic to read breakpad dumps to a separate class hierarchy

Also add unit tests.

BUG=253351
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/66333003

git-svn-id: svn://svn.chromium.org/blink/trunk@161718 bbb929c8-8fbe-4397-9dbb-9b2b20218538