Use 10Mb chunks when writing heap snapshot to temp file

This speeds up writing to the temp file about 15x

BUG=324769
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/107593004

git-svn-id: svn://svn.chromium.org/blink/trunk@164319 bbb929c8-8fbe-4397-9dbb-9b2b20218538