IncrementalMarking has nine bytes of padding in 32-bit and 64-bit
builds. Fixing 32-bit builds just requires moving the one-byte
incremental_marking_job_ member. Fixing 64-bit requires moving the
four-byte state_ member. This change reduces the padding to one byte.

On 64-bit this reduces its size from 152 to 144. This also fits heap
granularity better. On 32-bit it goes from 96 to 88 bytes.

The initial padding was found with llvm-pdbdump.exe. The fix was
verified by compiling v8/src/assembler.cc with the undocumented
/d1reportSingleClassLayout option, like this:

    /d1reportSingleClassLayoutIncrementalMarking

The savings should apply on all platforms, or at worst should make no
difference except for improving alignment.

Thanks to zturner@ for some llvm-pdbdump improvements.

BUG=chromium:710933

Review-Url: https://codereview.chromium.org/2808473003
Cr-Commit-Position: refs/heads/master@{#44698}