[tracing] Simplify design of MemoryAllocatorDump

[tracing] Simplify design of MemoryAllocatorDump

This is a follow-up to crrev.com/1084213003 (which introduces the
MemoryAllocatorAttributesTypes) and crrev.com/1093953002 (which
introduces the shared MemoryDumpSessionState).
The design of MemoryAllocatorDump is simplified, as follows:
MemoryAllocatorDump(s) are uniquely identified witin a
ProcessMemoryDump by an allocator_name and a heap_name.
The former acts as a type, and determines the types
of the attributes. The latter defines the rest of the dump name.
As far as the trace is concerned, the allocator dumps will be
identified the trace by an absolute path, that is:
allocator_name/heap/subheap.

BUG=466141
Committed: https://crrev.com/ba8859465a0ff1a6b9eb63f9ed5d4cd8bba69cfb
Cr-Commit-Position: refs/heads/master@{#325994}

[Primiano Tucci (use gerrit), 2015-04-18 18:56:10]

primiano@chromium.org changed reviewers:
+ nduca@chromium.org

[Primiano Tucci (use gerrit), 2015-04-18 18:56:10]

PTAL, This is a set of 3 CLs:

crrev.com/1084213003: Introduce the MemoryAllocatorAttributesTypes
crrev.com/1093953002: Introduce the MemoryDumpSessionState
[this cl] crrev.com/1095003002: final plumbing

[nduca, 2015-04-19 18:01:53]

lgtm

[Primiano Tucci (use gerrit), 2015-04-21 06:10:44]

The CQ bit was checked by primiano@chromium.org

[Primiano Tucci (use gerrit), 2015-04-21 06:10:45]

The patchset sent to the CQ was uploaded after l-g-t-m from nduca@chromium.org
Link to the patchset: https://codereview.chromium.org/1095003002/#ps60001
(title: "Rebase")

[commit-bot: I haz the power, 2015-04-21 06:10:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1095003002/60001

[Primiano Tucci (use gerrit), 2015-04-21 06:11:32]

The CQ bit was unchecked by primiano@chromium.org

[Primiano Tucci (use gerrit), 2015-04-21 06:12:38]

The CQ bit was checked by primiano@chromium.org

[commit-bot: I haz the power, 2015-04-21 06:12:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1095003002/60001

[commit-bot: I haz the power, 2015-04-21 07:08:25]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-04-21 07:09:17]

Patchset 4 (id:??) landed as
https://crrev.com/ba8859465a0ff1a6b9eb63f9ed5d4cd8bba69cfb
Cr-Commit-Position: refs/heads/master@{#325994}